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.
LcdSpinBox: added displayOffset property

The new displayOffset property is an offset which is always added to the
value of model so we can display values in a user-friendly way if they
internally start at 0.
InstrumentMidiIOView: rebase all values in SpinBoxes to 1

To keep consistency rebase all displayed values to 1 like we had for
MIDI channel and program already.
EffectRackView: added layouts for widget management

Also tweaked overall appearance a bit so that we have less nested frames.
New style & gfx for master vol/pitch sliders, fx front panel artwork