slider: propagate the "disabled" property value to internal children
slider: propagate the "disabled" property value to internal children

Make sure the internal children have the same value of the "disabled"
property so they are consistent with the rest of the widget.

http://bugzilla.clutter-project.org/show_bug.cgi?id=2470

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