unlockDialog: Set accessible name of icon-only buttons

We turned both the auth prompt's cancel button and the switch user
button into icon buttons now, which means they are completely cryptic
when using a screen reader.

Just use the previously used labels as accessible names, which has the
nice side effect of lowering the impact of the string freeze break.

https://gitlab.gnome.org/GNOME/gnome-shell/issues/2210

js/ui/unlockDialog.js

@@ -555,6 +555,7 @@ var UnlockDialog = GObject.registerClass({
         // Switch User button
         this._otherUserButton = new St.Button({
             style_class: 'modal-dialog-button button switch-user-button',
+            accessible_name: _('Log in as another user'),
             can_focus: true,
             reactive: true,
             x_align: Clutter.ActorAlign.END,