[Etoys] New Paint brush feature.

Hello,

I added new feature for paint tool in 1041DrawBackground-tak.cs. You can draw something on background of existing painting. It is effective to mimic like water color painting. It would help contents author rather than children. It was one of Alan's request list.

- Start paint tool by brush icon in Navigator flap or repaint handle. - Choose color and brush size. - Draw something. - Choose another color. - Draw something with Shift + drag.

I understand such Shift + drag is bad and unsophisticated. It should need proper UI in PaintBox widget. But the deadline for OLPC is very soon, so I gave up to put new button in there now.

Cheers, - Takashi