Only create MIDI regions on click when in draw mode.

It was much too easy to accidentally create MIDI regions in object mode.  If
the user isn't in draw mode anyway, then even after creating a region, they
can't put notes in it, so I don't think we've lost any discoverability here.
Fix MIDI region keyboard editing (add key handling to canvas).
Fix entering/editing MIDI region "automation" (controllers).

Two issues:

1) AutomationRegionView not receiving events (CC breakage).

2) Entered values must be rounded to integer for MIDI controllers.  This should
be done more generically with ParameterDescriptor for all controls (also
supporting boolean toggles and such), once ParameterDescriptor becomes more
pervasive and takes over Evoral::Parameter, but this will do for now.