Add a configuration for unfocused window transparency in Alt+Tab

Currently the opacity value of unfocused window when the main window is highlighted (in pressing Alt+Tab) is hard coded (0.15) in effects/highlightwindow/highlightwindow.cpp.

I wonder if we can add a configuration for this value. Some users (like me) may prefer the old style Alt+Tab without transparency effect, but still want to use a compositor (or Wayland).

I can write a patch if you are interested. I think I can just add a new configuration variable in kwinrc like ShowTabBox, without a GUI input box in kcm. Let me know if you think differently.