The destroy vfunc might be called during object finalization, and
we can't call any JS from a GC finalizer, so we use a signal
connection instead, as that is removed by GObject the first time
the object is disposed.
https://bugzilla.gnome.org/show_bug.cgi?id=719730
The user widget is the username and avatar shown on
the unlock dialog.
The login dialog has something very similar.
This commit separates the user widget out to its own
file, so we can use it from the login dialog in a
later commit.
https://bugzilla.gnome.org/show_bug.cgi?id=694062
