[NbtkWidget] add back  to nbtk_stylable_iface_init and use it
[NbtkWidget] add back  to nbtk_stylable_iface_init and use it

Set is_initialized to TRUE to prevent intialisation multiple times

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