We were cleaning up self._groups, but the actors for all previous
groups/layers/modes would remain attached to the aspect container,
simply hidden.
Under some circumstances this can really make the amount of actors
in the shell stage to quickly ramp up, it's not just a "leak" but
also has potential side effects on performance.
We should destroy all child actors of this._aspectContainer, except
the static ones (emoji and keypad).
While at it, fix this._groups re-initialization, as it's actually an
object, not an array.
https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/523
Closes?: https://gitlab.gnome.org/GNOME/mutter/issues/556
(cherry picked from commit ed999ce92693b8f1ab4040444a3e6e380e7b4475)
5100458c1b