We still need to possibly scan the toplevel after selecting
a widget, since we don't monitor the list of toplevels, so
the window may not be in our tree yet. This was broken in
commit e7bd73f2b0.
What was happening that the button press event that we use
to select a widget caused another clicked signal emission of
the inspect button, starting another pick action.
In contrast to the flashing, where we blink the widget a
few times, this is explicitly turned on and off.
It will be used for indicating widgets that are part of
a size group, in the next commit.
Moving the inspector into libgtk lets use reuse internals without
having to add public API for everything or inventing awkward private
call conventions.
https://bugzilla.gnome.org/show_bug.cgi?id=730095
