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.
ViewBackground never unsubscribed from UIConfiguration
callbacks when it was destroyed. Loading another session
that can cause UIConfig change signals to be emitted
caused a heap-use-after-free.
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.
