1. use an idle callback
2. MidiViewBackground::apply_note_range() now returns true/false
depending on whether or not it was able to make the requested change.
It might fail due to out of bounds note line size, for example.
This also uses the new API exposed by MidiViewBackground so that the header simply re-uses
the same exact position & height values computed by the background for its note lines.
PianoRollHeaderBase is the base class with the shared implementation.
PianoRollHeader is the GTK widget derived class.
ArdourCanvas::PianoRollHeader is the canvas item derived class.
Both PianoRollHeader implementations have to implement various methods that gloss
over the differences between drawing and event handling in a GTK widget context
or in a canvas item context
