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
