loginDialog: fix reactivity of first user in user list

After the login banner is shown and hidden, the first user
in the user list becomes non-reactive.  This is because the
banner is given an opacity of 0, but still allocated.

This commit fixes that by hiding the banner explicitly.

https://bugzilla.gnome.org/show_bug.cgi?id=743370

js/gdm/loginDialog.js

@@ -709,6 +709,7 @@ const LoginDialog = new Lang.Class({
     },
 
     _fadeInBannerView: function() {
+        this._bannerView.show();
         Tweener.addTween(this._bannerView,
                          { opacity: 255,
                            time: _FADE_ANIMATION_TIME,
@@ -718,6 +719,7 @@ const LoginDialog = new Lang.Class({
     _hideBannerView: function() {
         Tweener.removeTweens(this._bannerView);
         this._bannerView.opacity = 0;
+        this._bannerView.hide();
     },
 
     _updateLogoTexture: function(cache, file) {