Instead of manually doing it everywhere, just call set_object (NULL)
from the selection_changed handler. Fix all the set_object() functions
to deal with NULL.
Nice to have a quick way of testing this everywhere.
The implementation is not quite perfect: due to the way text
direction works in GTK+, widgets that appear in the inspector
window while we are flipped will inherit the flipped direction
instead of the fixed direction of the inspector window.
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.
