Not needed yet.

This gives a more complete picture.

These are interesting objects that are worth displaying in the tree.

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.

The tree contains other objects besides widgets, so label it
'Objects'.

The code for dealing with object-valued properties had several
bugs that lead to crashes. This change makes it work.

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.

Better to talk about GtkInspector in the error messages, too.

There is no reason not to do this.