overviewControls: Use a custom layout to catch all geometry changes

We currently update workspaces geometry when we are notified about
allocation changes of the overview group; however as the geometry
is based on stage coordinates, we miss notifications when the
allocation relative to the parent is unchanged, which happens when
the primary monitor's position changes but not its resolution.
Use a custom layout manager to give us a signal that is emitted
reliably.

https://bugzilla.gnome.org/show_bug.cgi?id=708009