[sugar] API deprecation

[Marco Pesenti Gritti]

Hello,

the Activity window now contains an HippoCanvas by default.

I updated the Toolbar snippet to reflect this:
http://wiki.laptop.org/go/Sugar_Code_Snippets

Activities that are still using gtk widgets instead of the hippo based
controls should hippo widget embedding. Something like this:

widget_item = hippo.CanvasWidget(widget=activity_main_widget)
self.set_root(widget_item)

I added an hack to not break activities which are still using the old
API, though it will be removed after we have built the stable image.

Marco