Issues that need to be fixed for 17.12 release:
[x] copy / paste clips
[ ] project duration
[ ] composition : move a_track correctly
[ ] geometry keyframes (transform,etc)
[ ] slowmotion effect - in progress
[ ] fix adding clip in bin (corruption with color clip id because of unfinished porting to new method)
[ ] split audio
[ ] define target track (audio/video) for operations
[x] Clicking away (eg at an empty place of the timeline) should clear clip selection
[ ] Allow compositions in groups
[ ] Copy/paste compositions
[ ] Insert several clips