slider: set the "focus" pseudo class on the slider
slider: set the "focus" pseudo class on the slider

Set the "focus" pseudo class on the slider, rather than the children. This
simplifies writing focus style rules for different parts of the slider.

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