[NbtkBin] remove padding property implementation and use NbtkWidget's
[NbtkBin] remove padding property implementation and use NbtkWidget's

NbtkWidget now installs the padding css property, so we use that
implementation rather than our own.

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