popupMenu: don't handle key presses directly if there are modifiers

popupMenu: don't handle key presses directly if there are modifiers

commit 1d58ea25ab6f0472bdeffc9170ece37d5dd71e64 changed the
popupMenu code to handle key presses on source actors directly.

This fixed keynav into the session gear menu at the login screen.

Unfortunately, it also causes spurious pop ups if a modifier happens
to be held down.

This commit checks for modifiers and short circuits the up front
key handling if any are found.

https://bugzilla.gnome.org/show_bug.cgi?id=740043