widget: Forward focus_highlight_style to win

widget: Forward focus_highlight_style to win

See the previous commits. All focus_highlight APIs are defined in the
Widget class but only implemented at the Window level. For consistency I
believe this call should also be forwarded to the window. The previous
logic had absolutely no effect at all, except storing a stringshare.

The day focus_highlight becomes widget-specific (i.e. each widget has
its own highlight style) then this can be changed.

Note: This will apply to legacy API as well.

Ref T5363