In a normal user session you can't have more than one
popup, because the popup is modal and we don't allow
the popup to show up when there are other modals.
In a GDM session, however, the login dialog is modal, and
we want a popup, so we don't have that same check.
This commit changes the ctrlAltTab manager code to not
allow multiple popups.
https://bugzilla.gnome.org/show_bug.cgi?id=659177