The sidebar in the object page was using up more space than it deserves.
Replace it with a combo box in the subheader. Now that we have more space,
put the CSS node tree and the style properties in the same page again.
We know the objects in a size group are always widgets, so we
can avoid hard-to-track down problems with weak references by
just cleaning up when the object gets destroyed. There is still
a chance that we show a widget as part of the group after it
has been removed, but size groups simply have no signals that
would let us avoid that.
Add a tab that shows size groups of a widget. The properties
of the size group are available here, as well as the widgets
that are part of the size group. We highlight the widgets in
the application when their row in the inspector is hovered.
