[NbtkBoxLayout] use NbtkBoxLayoutChild as child meta type
[NbtkBoxLayout] use NbtkBoxLayoutChild as child meta type

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