Support setting the SizeConstraint for WidgetGroups.
Re-implement WWidgetStack sizeHint() and minimumSizeHint() because
QStackedWidget's implementation is not what we want.
Add NULL control checks in WWidgetStack.
Make widgets in a WidgetStack hide when their trigger control is set to 0.
Get rid of custom logic for new-style skins and just parse them as a WidgetGroup.
Parent WWidgetGroup to QFrame instead of QGroupBox. Rewrite style selectors using QGroupBox to use QFrame.