Xfdesktop should allow only one instance of its context menu

This is probably only a side-effect problem in practice, as in #182 (closed) where the problem had to be fixed in Libxfce4ui, or in xfce4-panel#585 (closed) where it had to be fixed in Xfce4-panel.

But it remains that Xfdesktop should not allow the display of a new context menu when there is already one displayed.

It is still possible to recreate this problem with a loop on xfdesktop --menu as mentioned in #182 (comment 42225).