Big update this time around!
* Cleaned up mouse coordinates: all non-deprecated functions get mouseover coordinates in widget-local space
* Cleaned up mouse event delivery: the recipient of the mouseDown event is guaranteed to receive all related mouseDrag and mouseUp events
* Added optional mouseover coordinates to the paintBackground method of widgets
* Added WScrollBar, which manages progress across a windowed area
* Added WListPanel, a scrollable list resembling an android recycler view
* Added WSlider and WLabeledSlider for selecting a value from within a range
It's recommended to stop using paintForeground, at least for now, because the reported mouse coordinates could change at any time in the future to local ones.