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.
Add a tab that shows available signals for each object. For now,
we only show if each signal has handlers connected or now. More
functionality will be added later.
The list of toplevels also includes hidden combobox popups
and the like, so we have to be a little careful. To ensure
the right choice, we now pick the first visible window
that is not a GtkInspectorWindow.