#84 Theme aware UI-functions

[M W]

Hi

We need a function to clear the screen to the default system background (could be used at the entry to the resize/redraw handler in the canvas), also to get the system's default font color (it should already BE the default when no color given). Finally, a function to display the highlight rectangle would be nice.

This would allow to design an application interface built on the Canvas and still in the overall "theme" of the phone's interface. It would free the programmer from the dependence to the few default UI controls exposed to python, by writing custom controls IN python.

Counting on Your positive evaluation.