A way to get the size of layout

[Nedok]

Currently it's quite hard to get the bounding box of a layout. (Top left and bottom right points.)
This is useful for placing backdrops for dynamic generated GUI (like item lists), as well as for creating scroll enabled GUI.

A solution could be to add a boundingSize() function to layout that return x, y, width, height where x,y points to the absolut top position of the layout.