loginDialog: Add missing return value

_loadUserList() may be used as idle handler, so we should explicitly
return a boolean.

https://bugzilla.gnome.org/show_bug.cgi?id=725905
loginDialog: Defer loading user list until needed

Loading the user list can be expensive, for instance when there is
a large number of users and/or their avatars have to be fetched over
the network. In case the user list is disabled anyway, there is no
point in doing that work just to hide it, so stop doing that.

https://bugzilla.gnome.org/show_bug.cgi?id=725905