NbtkWidget: Unparent the texture frame for the background on dispose
NbtkWidget: Unparent the texture frame for the background on dispose

This prevents us from leaking the NbtkTextureFrame that is used to paint the
background image.

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