Tile: Calculate children-of-children in teh childCount
Otherwise we count a tile that has sub-tiles as 1, which would make handleInteractiveMoveResize think there are no areas when there are.
@mart note this is because it's stored as:
tiles={"layoutDirection":"horizontal","tiles":[{"layoutDirection":"vertical","tiles":[{"height":0.5},{"height":0.5}],"width":1}]}
it should probably be:
tiles={"layoutDirection":"vertical","tiles":[{"height":0.5},{"height":0.5}],"width":1}