freeze break request for gnome-shell

[Matthias Clasen <matthias clasen gmail com>]

 See https://bugzilla.gnome.org/show_bug.cgi?id=684459
I'd like to get a very small patch committed.

It addresses a problem related to user switching. When clicking the
'Switch user' menuitem, we switch vts, which causes the shell to stop
whatever it is doing. When we later switch back, the screen has been
locked (ie the shield is down), but the user menu is still showing -
on top of the shield, and over at the other edge of the screen. This
is very noticable (people will be playing with the shiny new lock
screen and discover it), and arguably some sort of privacy leak.

The patch makes a simple change: unpost the menu before initiating the
vt switch.

I already gave my +1 in the bug, looking for a second approval here.