Loop playback update and improvements

Different changes :
- Fix the loop shortcuts states to avoid being active in edit mode/lyrics
- Moved the loop In/Out positions in score instead of Scoreview
- Add loop on selection
- Change the way the shortcuts work (no checkable state for the loop In/Out buttons)
- Revert to the previous way of inserting the ME_LOOP event (using lower_bound causes a segmentation fault)