overviewControls: Update work area also on monitors-changed

The cached work area, which is the work area of the primary monitor,
effectively depends on two properties - the (global) work area and the
primary monitor - and we are only tracking changes to one of them. Also
track monitor changes to also cover the second case.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2623>

js/ui/overviewControls.js

@@ -50,6 +50,9 @@ class ControlsManagerLayout extends Clutter.BoxLayout {
         global.display.connectObject(
             'workareas-changed', () => this._updateWorkAreaBox(),
             this);
+        Main.layoutManager.connectObject(
+            'monitors-changed', () => this._updateWorkAreaBox(),
+            this);
         this._updateWorkAreaBox();
     }