userportal: Fix nasty issue in MenuPanelPopup

When running UserPortal in web (compiled) mode, a very
nasty issue occurs in MenuPanelPopup constructor. This
issue occurs *only* in web mode and causes UserPortal
login screen to hang after successful authentication.

JS stacktrace points to MenuPanelPopup using UiBinder:
- UiBinder-generated code does SimplePanel.add(widget)
  as per <g:SimplePanel> in MenuPanelPopup.ui.xml
- for some reason, 'widget' is an instance of
  DecoratedPopupPanel, instead of expected PopupPanel
- JS TypeError occurs with following message:
  DecoratedPopupPanel.decPanel field is not initialized

This patch fixes this issue by removing UiBinder usage
from MenuPanelPopup. This isn't a big deal, since
MenuPanelPopup uses UiBinder *only* to apply CSS styles,
its widgets are declared using @UiField(provided=true).

Change-Id: Ic3f9c3318234bb5315f950c0cba44e6eaa8cb24f
Bug-Url: https://bugzilla.redhat.com/992960
Signed-off-by: Vojtech Szocs <vszocs@redhat.com>