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
