popupMenu: Don't handle key presses directly if there are modifiers
Key events involved in a keyboard shortcut are not completely consumed by
Mutter. That means that if the popupMenu is bound to a shortcut (e.g.
Alt<Space>) and the user keeps the keys pressed, the same key-event will be
delivered to the popupMenu. We can workaround this issue filtering out all the
events where a a modifier is down (except capslock).