Currently when removing a space on the timeline, the user hits a keyboard shortcut (G for me, I forget what the default is) and then is presented with a dialog asking them to select which track to apply the action to. 
This makes for tedious and repetitive action when removing a lot of spaces on a particular track - I'm always looking for ways to replace any actions I perform with the mouse to actions performed with keyboard shortcuts, as it always results in a increase in speed.

What I would suggest as a fix to this issue would be to keep the defaults as they are (the dialogue helps new users understand what they are about to do and doesn't confuse them) but include a checkbox in the preferences called 'use selected track when Removing Space'. 

This way, whichever track is currently highlighted will always be the one which has space removed, and people who use 'remove space' a lot can do so much faster without having to constantly reach for the mouse and select the same answer in a dropdown menu. 

(you might also include a note in the track selection dialog saying 'track selection preferences can be set in Kdenlive's preferences' or something similar, so that people who also find this frustrating will discover that there is a fix available.)

Reproducible: Always

Steps to Reproduce:
1. place cursor in a gap on the timeline
2. hit shortcut for 'remove space'
3. dialog window pops up
4. reach for the mouse
5. select correct track
6. hit ok.
7. space is removed
8. repeat for as many spaces as you need to remove.


Expected Results:  
1. use the up/down arrows to select the track in which you will be working.
2. place cursor in a gap on the timeline
3. hit shortcut for 'remove space'
4. space is removed.
5. repeat for as many spaces as you need to remove.