This is pretty damn nice, now I can move and resize entire groups of widgets
together. Diagonal group resizing feels a bit strange but I wasn't expecting
it not to. :^)
It's now possible to edit widget properties inline in the properties window.
We're currently relying on the basic GVariant conversion functions to do
all the "parsing" but that's not gonna be good enough.
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. :^)
