Improve zOrder

[p0nce]

• [ ] zOrder Wren access, read and write, like .visibility
• [ ] zOrder should call setDirtyWhole if there is a change
• [ ] setZOrder should be deprecated in favour of a zOrder property
• [ ] A widget can indicate with a UIElement flag that it completely writes to the dirtyRects opaquely (no transparency, no partial update). This way, we could perhaps avoid drawing widgets below. Presumably this flag can change at runtime, but in a first approximation it should be disabled. Separate flags for both PBR and Raw.