window: Add 'child' property
window: Add 'child' property

MxWindow had get/set functions for the window child, but this was not
reflected in a property. Add this property.

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