workspaceAnimation: Notify 'progress' property properly

The getter of the MonitorGroup's 'progress' property sets
some values based on it, but doesn't notify the property
change.

Fix that by calling this.notify('progress') when the setter
is called.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2881>

js/ui/workspaceAnimation.js

@@ -214,6 +214,8 @@ const MonitorGroup = GObject.registerClass({
             this._container.x = Math.round(p * this.baseDistance);
         else
             this._container.x = -Math.round(p * this.baseDistance);
+
+        this.notify('progress');
     }
 
     get index() {