new attempt to introduce a playback popover

This is yet an other attempt at introducing a playback popover. A lot of code has changed since !47 (especially player.py). It is now way easier to introduce this feature.
Except from some ui files, the code is really different from the previous attempts. I think that all the previous remarks from !47 are resolved.

I hope this proposal does not bypasses an other work on it. At first, I just wanted to check that the rewrite of player.py was good enough.

I'm not very satisfied with the way a transition happens when repeat mode changes or a new song is selected. Some additional work or suggestions may be needed.