[NbtkBoxLayout] Implement the "expand" child option
[NbtkBoxLayout] Implement the "expand" child option

Allow children marked as "expand" to be allocated any extra space left
over from allocating all children at their preferred size.

pwithnall/mx,jonnylamb/mx,clutter-project/mx,clutter-project/mx,pwithnall/mx,pwithnall/mx,jonnylamb/mx,jonnylamb/mx,pwithnall/mx,clutter-project/mx,clutter-project/mx,jonnylamb/mx