If modes provide a QMenu, a little arrow is shown next to the mode icon.
If the user clicks there, the menu is shown instead of changing the
mode.

Limitations: Modes need to provide some QMenu instance already when they
are added to the object pool. Setting or removing the menu later will
not update the UI.

Change-Id: Ic4ef709e6200afcff14f41054a5dd98c37b0b849
Reviewed-by: hjk <hjk@theqtcompany.com>