unlockDialog: set password char on verif. failure

Commit c3cab28 removed bits setting the password char that was used to
allow typing the password after a failure but before the first question
for the new attempt came.

Without that code, in complex PAM setups you can end up with a visible
password.

https://bugzilla.gnome.org/show_bug.cgi?id=687113

js/ui/unlockDialog.js

@@ -341,6 +341,8 @@ const UnlockDialog = new Lang.Class({
         this._firstQuestion = true;
 
         this._promptEntry.text = '';
+        this._promptEntry.clutter_text.set_password_char('\u25cf');
+        this._promptEntry.menu.isPassword = true;
 
         this._updateSensitivity(false);
         this._setWorking(false);