keyboard: Update extended key size on parent size changes

Extended keys should have the same size as their parent key, so
make sure to update them when the parent size changes.

https://gitlab.gnome.org/GNOME/gnome-shell/issues/1976

js/ui/keyboard.js

@@ -460,6 +460,8 @@ var Key = GObject.registerClass({
             this._extendedKeyboard.add(key);
 
             key.set_size(...this.keyButton.allocation.get_size());
+            this.keyButton.connect('notify::allocation',
+                () => key.set_size(...this.keyButton.allocation.get_size()));
         }
         this._boxPointer.bin.add_actor(this._extendedKeyboard);
     }