The widget-tree was not safe against object just going away.
Fix this by using row references instead of iters where
necessary, and by using weak refs to clean up when objects
die.
It does not really add much over the sensitive/insensitive rendering
that is already indicating which widget is mapped. At the same time,
set up signal handlers so we can update that when the widget changes.
When showing the objects in the tree, use the property name
as the name thats shown in the list. This makes it easier
to differentiate e.g. hadjustment and vadjustment in a
GtkScrolledWindow.
