overviewControls: Incorporate ActivitiesContainer

overviewControls: Incorporate ActivitiesContainer

Move AppDisplay, WorkspacesDisplay, and ThumbnailsBox from ViewSelector to
ControlsManager. This allows to always allocate the correct size for AppDisplay,
and will enable for a plethora of further improvements. The end goal is to
completely remove ViewSelector, and let ControlsManager handle the layout of
everything that's visible in the overview.

For now, replace the apps page with a dummy actor in ViewSelector.

Adjust various callers around the codebase to not access the ViewSelector
directly from the overview anymore.

Bind the opacity of the primary workspace to WorkspaceDisplay's opacity. This
allows removing the parent opacity hack in place, which will be done by the
next commit.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1624>