Fix MIDI note cut/copy/paste.
Fix missing return statement in deleted region scenario.
Implement "multi-paste" for notes, regions, and automation.

The idea here is that pasting several times to the same location doesn't make
sense.  Instead, the paste is appended past the last paste, snapped to the
grid.  This make it simple to replicate a given section a number of times,
simply by copying once and pasting several times.

This behaviour only appears when successive pastes are done to the same
location (whatever the edit point is).  When the paste point changes, the
"multi-paste" state is reset.

Boots 'n cats 'n boots 'n cats.