Refactor the core drawing flow.

1) delete the background attribute from main.xml or our drawing want show, screen will always be the background.
2) let all direct drawing via PainterThread and all drawable object passed to it be created with PaintPerformer. I want PaintPerformer do all logic associate with drawing, and PainterThread's job is only to draw drawable objects.
3) signal class need refactoring to support reuse.
4) paint instance associate with it's container drawable object need refectoring.
5) PaintThread need refactoring to use Bitmap when drawing object, because terrible flash will occur when finger move on screen.