removing some WF/Drawing code in BSplitterPanelImpl.

Though there was not much. Since its implementation would rather depend
on the backend, I'd leave this almost as is.
simplify DraggableButton.
add ApplicationDoEvents() in common UI Fx. Reduce winforms-isms on controls.
PictOverview is mostly moved to "model" (which means plat-neutral code).
Fix BSplitContainer public member XML deserialization etc.
remove winforms-isms from PictPianoRollImpl.
moved most of PictPianoRoll code to "model".
some superficial changes in ProgressBarWithLabel.
removed winforms/drawing reference in PropertyPanelContainerImpl.
move some PropertyPanelContainerImpl code to model.

This could be implemented by NOT using XML... but it's not too much anyways.
remove winforms/drawing references in PropertyPanelImpl.
most of PropertyPanel code is now moved to "model"