Allow -1 values for minimum/maximum width/height (interpret as 'do not set').
Support <MinimumSize>, <MaximumSize>, <SizePolicy>, and <Size> for the root <skin> node.
Support putting children of <skin> node in a <Children> block (but also support the current approach for backwards compatibility).
Support specifying a layout for the root <skin> widget with <Layout> similar to WidgetGroups.