Only send active window changes to X11 root window if the X11 window changed
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.
Verified with xev -root that the active window does not get
Subscribers: plasma-devel, kwin
Differential Revision: https://phabricator.kde.org/D7096