ljglibs: Add some for Gdk.{Screen, Window}
ljglibs: signal.unref_handle returns the unrefed callback

Useful for maintaining a reference outside.
ljglibs: Add some support needed for the Howl popups
Remove last lgi usages from Howl
Remove LGI from build
Make editors collectable

Now it's time to reap one of the benefits from switching away from LGI:
Editors are now garbage collected as they should.