Bug 474414

SUMMARY
Audio clips don't behave as expected when imported in clip explorer, nor in the clip monitor when they don't have a thumbnail attached.
Regular audio clips can't be added to the timeline anymore, I am 100% certain I could do that in previous versions of Kdenlive, to me it looks like a regression.

Note : I'm using a laptop that uses a hybrid GPU (Intel integrated and NVIDIA 1050 Ti working altogether) and the clip preview is generated using the default profile (Automatic NVIDIA).

STEPS TO REPRODUCE
1. Start a new kdenlive instance (default project)
2. In the clip explorer, import a regular mp3 for which you have a thumbnail (embedded in the mp3 metadata) -> This one will get both the thumbnail calculated and its waveform will show in the clip monitor when selected in the clip explorer
3. Still in the clip explorer, import another mp3 without any thumbnail (like a sound made in audacity, could be .ogg or .wav, same issue) -> this one's calculated preview will stay blank and nothing will show in the clip monitor.
4. Try to add the 1rst MP3 to the timeline -> This one works, but the attached thumbnail is now showing in the video track and the audio in the audio track. But as this is an audio file, I don't want the thumbnail to be added in the timeline...but it's another issue, probably parallel to this one
5. Try to add the 2nd MP3 (the one without the thumbnail) to the timeline -> this one can't be added ! Furthermore, when kdenlive is launched from the command line, the following log show repeatedly : 
```
clip not ready or not compatible PlaylistState::VideoOnly true
clip not ready or not compatible PlaylistState::VideoOnly true
clip not ready or not compatible PlaylistState::VideoOnly true
clip not ready or not compatible PlaylistState::VideoOnly true
clip not ready or not compatible PlaylistState::VideoOnly true
clip not ready or not compatible PlaylistState::VideoOnly true
clip not ready or not compatible PlaylistState::VideoOnly true
```

OBSERVED RESULT
Audio files with an attached thumbnail can be added to the timeline and despite the thumbnail showing for the whole length of the song, we can still split it later on (it's just a bit annoying).

The audio file that doesn't have a thumbnail attached (regular audio file) won't be previewed in both the clip explorer and clip monitor, and this one cannot be added to the timeline.


EXPECTED RESULT
As long as the input audio format is compatible with kdenlive, audio files shall be listed as audio files, and their waveform might be previewed both in clip explorer and clip monitor. We shall add them easily to the timeline like any other clip, and when added in the timeline, I don't expect their potential thumbnail to be displayed (or maybe have a dialog window that lets you decide whether you want the thumbnail as a video or not, should default to audio only - no thumbnail).


SOFTWARE/OS VERSIONS 
Linux : Arch Linux (uname -a -> Linux arch-linux 6.1.51-1-lts #1 SMP PREEMPT_DYNAMIC x86_64 GNU/Linux)
Desktop manager : GNOME 3 
Qt Version: 5.15.0
MLT 7.18.0

ADDITIONAL INFORMATION
Reproduced in version 23.04.xx, 23.08.xx and up until commit #55f1cf34347ba784ae084aa990adcba635b7361c on git-master (local build)