A couple of people have walked up to me and asked how to get to the
unlock screen from the screen shield. This was partly addressed by
bug 682285, but all three people who asked me about this said they
tried the return key and were surprised when it didn't work.
It sort of makes sense, since the user is "enter"ing the computer or
"return"ing to it.
This commit makes enter work in addition to the existing escape key.
https://bugzilla.gnome.org/show_bug.cgi?id=683889
4254fa3d38