Fix and simplify step_mouse_mode.
Unify gain and draw tools.

There might be a few special cases lingering around from attempts at magic tool
business, but this seems to work fine as far as I can tell.
Draw region automation with draw in either mode.

Perhaps debatable if contents should be edited when not in internal mode
whatsoever, but consistent with audio region gain and track automation.  It's
less of a problem with the draw tool than, say, object since drawing stuff is
its entire purpose.
Fix uninitialized value in note create drag.
Fix alignment of automation paste.

Preserve alignment between notes and control points when doing an internal
copy/paste of both.

Relative alignment between points on multi-paste is still not preserved.  Both
behaviours here are actually useful, perhaps a modifier...