UI - Fix serialize/deserialize of child Quads in QuadRenderable.

[MakoEnergy]

Different assumptions are being made in different places in UI code. QuadRenderables create new children and those children to themselves when deserializing child quads, but don't clear already existing quads prior to this.

Additionally, the Scrollbar class will check after child quads are deserialized if expected children are present and map them to the more explicit pointers they are holding for those children. Other Widgets with similar functionality (DropdownList, ListBox, Spinner) do not do this.

They should all use a common idiom based on a solidified policy set by the QuadRenderable class.