Bug 486394

# Description
Hi Kdenlive team,

Reporting for commit [db49d96d89858e3d1af19fd3346c5e5771b1d5ae](https://invent.kde.org/multimedia/kdenlive/-/commit/db49d96d89858e3d1af19fd3346c5e5771b1d5ae/pipelines?ref=master)

I noticed that when converting a project that has still image clips from the HD 1080p 25fps profile to the HD 1080p 30fps profile, still image clips duration is lost.
This can be seen in the .kdenlive project file.

Here is a sample of what I'm seeing in the project file:

Before the conversion
```xml
<producer id="producer7" in="00:00:00.000" out="00:00:04.960">
  <property name="length">00:00:05.000</property>       <!-- Length here is expressed in "time" -->
  <property name="eof">pause</property>
  <property name="resource">PANO_20230811_135645.jpg</property>
  <property name="ttl">25</property>
  <property name="aspect_ratio">1</property>
  <property name="meta.media.progressive">1</property>
  <property name="seekable">1</property>
  <property name="format">1</property>
  <property name="meta.media.width">15920</property>
  <property name="meta.media.height">4288</property>
  <property name="mlt_service">qimage</property>
  <property name="kdenlive:duration">00:00:05:00</property>  <!-- Here the duration property is lost as well !-->
  <property name="xml">was here</property>
  <property name="kdenlive:folderid">17</property>
  <property name="kdenlive:id">16</property>
  <property name="kdenlive:clip_type">2</property>
  <property name="kdenlive:file_size">37679621</property>
  <property name="kdenlive:file_hash">10aa9ff46e760b15eb24dbabe6514047</property>
  <property name="kdenlive:monitorPosition">0</property>
 </producer>
``` 
 
 And here after the conversion
 ```xml
 <producer id="producer7" in="00:00:00.000" out="-1">  <!-- Here the in/out duration are not recomputed correctly !-->
  <property name="length">0</property>		       <!-- Length goes down to 0 -->	
  <property name="eof">pause</property>
  <property name="resource">PANO_20230811_135645.jpg</property>
  <property name="ttl">25</property>
  <property name="aspect_ratio">1</property>
  <property name="meta.media.progressive">1</property>
  <property name="seekable">1</property>
  <property name="format">1</property>
  <property name="meta.media.width">15920</property>
  <property name="meta.media.height">4288</property>
  <property name="mlt_service">qimage</property>
					    	       <!-- Here the duration property is lost as well !-->
  <property name="xml">was here</property>
  <property name="kdenlive:folderid">17</property>
  <property name="kdenlive:id">16</property>
  <property name="kdenlive:clip_type">2</property>
  <property name="kdenlive:file_size">37679621</property>
  <property name="kdenlive:file_hash">10aa9ff46e760b15eb24dbabe6514047</property>
  <property name="kdenlive:monitorPosition">0</property>
 </producer>
 ```
 
 In project sources, this image clip duration is now shown as being 00:00:00:25, hence 250 ms which is believe is some sort of default.

# STEPS TO REPRODUCE
1. Create a new HD 1080p 25fps project in Kdenlive
2. Drag n' drop an image clip in project sources -> image clip in project sources should be 5s by default
  2.1. -> drag n' drop the image clip in the timeline 
3. Save
4. Go to project/Project configuration and pick the **HD 1080 30fps** profile
5. Save again (file name should have changed to <filename>-3000.kdenlive
6. Open a difftool and compare <filename>.kdenlive and <filename>-3000.kdenlive
7. In Kdenlive, the image clip duration in sources should have changed to 00:00:00:25
  7.1. The image clip length in the timeline is now really short.
 
# OBSERVED RESULT
Image clip length / duration information is lost

# EXPECTED RESULT 
Image clip length remains unchanged

# Further remarks
The behavior is different for video clips (video "producers").
Video clips are encoded with their length expressed in Frames count and are then converted to "time"
E.g. :
Video clip before the conversion
```xml
<chain id="chain0" out="00:00:19.680">
  <property name="length">493</property>        <!-- Here the length is expressed using the frame rate -> This is a "frame" count -->
  <property name="eof">pause</property>
  <property name="resource">/home/groben/Images/Photos/Laos à vélo (août 2023)/Laos jour 5 - Paksé - Paksong/Vidéos/VID_20230811_153300.mp4</property>
  <property name="mlt_service">avformat-novalidate</property>
  <property name="seekable">1</property>
  ...
</chain>
```

After the conversion
```xml
 <chain id="chain0" out="00:00:19.680">
  <property name="length">00:00:19.720</property>       <!-- Now the length is expressed in time, note that there were time/frame conversion errors and losses and that now the "out" property doesn't match the length anymore-->
  <property name="eof">pause</property>
  <property name="resource">/home/groben/Images/Photos/Laos à vélo (août 2023)/Laos jour 5 - Paksé - Paksong/Vidéos/VID_20230811_153300.mp4</property>
  <property name="mlt_service">avformat-novalidate</property>
  <property name="seekable">1</property>
  ...
</chain>
``` 
=> I have had many issues with video conversion on a complex project with lots of resources and I can't reproduce it with a simple kdenlive project.
The symptoms in this case was that :
* Images clips duration was reset to 0
* Video clips duration was messed up and their overall length was reduced by 30/25 whereas their playback speed was increased by the same ratio !
  * Note:  Video clips were all around 30 fps by the way ; I'd expect Kdenlive to never alter length and playback speeds when changing profiles ; rendering a 30 fps video at 25 fps should only be regarded as downsampling. I think the end user should be in control of the actual playback speed.

## Extra remark (this would be another topic)
On linux, when entering the timeline of kdenlive, the bottom info bar show up hints such as :
"Shift + drag" for eraser selection, [...] "Windows" + "Alt" + drag to move [...].
+> Should we still use the term "Windows" for non-Windows platforms ?
 
# SOFTWARE/OS VERSIONS 
Linux/KDE Plasma: Linux archpc 6.8.7-arch1-1 (uname -a)
KDE dev environment version : 6.1.0
Qt Version: 6.7.0
MLT : 7.24.0

Thanks for your time :)