desktop_login: use esc not alt-f4 to dismiss menu on KDE
alt-f4 makes sense for an app (Terminal on GNOME) but not really
for closing the system menu (KDE). It seems like it worked till
a day or two back then broke, but I think just using Esc instead
rather than filing a bug is the best plan, I'm not sure I'd
*expect* alt-f4 to work for this.
Signed-off-by: Adam Williamson <awilliam@redhat.com>