In particular, keep the MidiViewBackground's sense of the data range intact,
use MidiModel::{highest,lowest}_note(), center ranges that don't fit
on the data range
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.
1. use a RectSet instead of a LineSet to draw note lines. Cairo provides
different coordinate -> pixel coloration rules for rectangles than for lines,
and it makes more sense in this case to use rectangles rather than fat lines.
2. change ::draw_note_lines() to ::setup_note_lines() since the method
doesn't actually draw anything.
This includes note heights, note positions, contents height
for streamviews, positions of tracks. It makes zero sense to think of these as
having potentially fractional positions.
In addition, fractional note heights and positions lead to numerous
errors drawing MIDI stuff at the pixel level.
This new inheritance heirarchy lets us share (a very small piece of) code
between streamviews in the Editor and the nascent piano roll object on the cue
page.
