[sugar] demo4 branch

[Marco Pesenti Gritti]

Hi,

The next demo is going to require big changes to the home page graphics.

I don't want to screw people that are writing activities though (the 
Activity API itself is going to remain pretty much the same).

So I created a branch for it. We can merge it back as soon as basic 
functionalities are working.

To check it out:

git checkout demo4

I also made HomeWindow use a goocanvas (empty right now) and changed 
screen resolution to what Walter said.

Marco