Loop playback update and improvements

[vgstef]

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)

[lasconic]

In seq.cpp; the loop events somehow has to be processed before the first note on event to avoid noise. Maybe a better solution would be to handle it in process() by sorting midi events of same tick.