switch gui to stateful widgets

[jamesoncollins]

Currently we flush the entire screen to black, and then redraw each screen element, for every frame. This is a waste since very few things change per frame.

Consider changing to a widget style, which is already available in uGFX, which can store teh state of the widgets and only update the parts of the widget that changed.

1. Don't clear to black
2. Widgets need to clear their now blank pixels
3. Widgets need to store their state.