So far KWin always updated the active window property even if the actual
window id hasn't changed. E.g. if a Wayland window was active and another
Wayland window gets activated the window id was and stays 0.
Nevertheless KWin updated the property causing wakeups in X server and
any application listening to property changes on the root window.
Futhermore this situation is an information leak: we leak when a Wayland
window gets activated to X11.
To solve this problem RootInfo caches the active window id and only
updates if it changes.