PianoRoll: improvements for note/panning widget

In short: make the volume/panning bars behave a bit more like knobs.
- Show the current value (volume/panning) as a textfloat when changing volume or panning
- Allow modifying the value with mouse wheel 
- Allow resetting to default (100% for volume, center for panning) with a middle-click
- Allow double-clicking to enter a value in text

There was some earlier functionality that had to be scrapped to implement the doubleclick functionality: doubleclicking the bar area used to clear the selection, but there are other ways to do that and I don't think it was that useful... but if it's needed, we could put it back so that it only happens when we're not near a note.