authPrompt: Don't ever ask for a username if smartcard service is in foreground

The smartcard service is put in the foreground in two cases:

1) If password service is disabled by admin configuratoin
2) if a smartcard is inserted

In either case we don't want to ask the user to pick a user from the
userlist.  We currently only avoid asking in case 2.

This commit fixes case 1.

https://bugzilla.gnome.org/show_bug.cgi?id=726263
layout: really queue region update before redraw

This code may have worked when written in 2009, but later gjs commit
b5e467d89aea43a8e32a1138d232c8a32e6b0785 removed the priority
parameter from idle_add.

Now, when running a constantly-updating client (es2gears) on an
embedded platform, _updateRegions() does not get called and I see
unresponsive window decorations.

Update the code to use meta_later_add() like other parts of the
shell, which is actually slightly better in this case anyway.
Solves the unresponsiveness problem.

https://bugzilla.gnome.org/show_bug.cgi?id=585500