[button] Update after the constification in ::draw-background
[button] Update after the constification in ::draw-background

The ClutterColor parameter in the ::draw-background virtual function
of NbtkWidget has been constified to avoid messing up the passed
background color.

NbtkButton is the only class overriding the default ::draw-background
and it does copy the color anyway.

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