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>