[Etoys] New Paint brush feature.

[Takashi Yamamiya]

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