ComboBox: added sizeHint() + coding style
TempoSyncKnob, Knob: allow construction without parent
InstrumentFunctionViews: manage widgets using layouts
Renamed instrument function classes and instances

Arpeggiator got InstrumentFunctionArpeggio and ChordCreator got
InstrumentFunctionNoteStacking. All related views and instances have
been renamed for better consistency.