Added note pencil tool, mock note adding (notes can be added visually but don't yet play).
Reworked MidiModel to be notes w/ duration instead of realtime style MIDI events.
Moved layering (stacked/overlaid) from auto time axis down to route time axis since it applies
to MIDI tracks as well.
git-svn-id: svn://localhost/ardour2/trunk@2128 d708f5d6-7413-0410-9779-e7cbd77b26cf