unlockDialog: move user widget into separate file

unlockDialog: move user widget into separate file

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