Track: allow resizing TrackContentObjects to less than one bar

As per popular demand, allow resizing TrackContentObjects to less than
one bar if appropriate subclass supports it.
InstrumentMidiIOView: added layouts for widget management

Instead of manually moving and resizing widgets, manage this using
appropriate layouts.
InstrumentTrack: set minimum size for save settings button

Set minimum size of 32x32 pixels for an appropriate appearance.