I need somewhere to centralize the knowledge about the different widget
types available. And VBProperty represents a property key/value of arbitrary
type (it uses a GVariant for the value.)
That first design was the wrong idea. Instead, have VBWidget instantiate
a GWidget of the appropriate type and parent it to the VBForm.
We then use a new "greedy hit-testing" mechanism in GWidget to prevent any
mouse events from reaching the VBForm's children.
To paint the grabbers above the child widgets, I added a slightly hackish
but kind of neat second_paint_event() that is called after a widget has
painted all of his children. :^)
