workspace-manager: Allow workspace layout to be overridden

meta_workspace_manager_override_workspace_layout is implemented by
calling meta_workspace_manager_update_workspace_layout which
respects the workspace_layout_overridden flag.  After the first call
to meta_workspace_manager_override_workspace_layout all subsequent
calls fail silently.

Reset workspace_layout_overridden to FALSE before calling
meta_workspace_manager_update_workspace_layout.

https://gitlab.gnome.org/GNOME/mutter/issues/270