workspacesView: Work around Clutter optimization

Clutter nowadays omits reallocations when only the stage position changed, that is when the allocation relative to the parent changed.

As a result (apart from better performance of course), workspaces in the overview may now end up with an outdated "actual geometry" in case the overview moved to a new primary monitor (of equal size as the previous one).

Work around that by emitting a signal from the overview on allocation changes, and use that to update the cached geometry.

We can revert that change once workspaces become part of the regular overview hierarchy.

Fixes #3211 (closed)