[NbtkTable] access child meta directly rather than using g_object_get
[NbtkTable] access child meta directly rather than using g_object_get

This brings a significant performance benefit when relayouting

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