slider: invalidate the style cache of internal children
slider: invalidate the style cache of internal children

Make sure the style of the internal children are updated when the style of
the slider changes.

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