fix start applications via pkexec through both the the menu launcher and ALT+F2

This will fix this bug in fedora.
https://bugzilla.redhat.com/show_bug.cgi?id=902010
Several system applications doesn't start from the menu but starting fine in a terminal.
It's already a known bug in gnome-panel,xfce-panel and gnome-schell.
https://bugzilla.xfce.org/show_bug.cgi?id=9373
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=690339
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=650038
https://bugzilla.gnome.org/show_bug.cgi?id=686059
The fix is adopted from gnome-panel and adjust for mate-panel.