[startkde] Move ksplash loading to handle new kwin_wayland

KSplash is changed to read which theme to use directly. It's already
linking config and it already loads the default shared config implicitly
so we weren't saving anything having it done by the launcher.

On X11 ksplash is still managed by startplasma-x11 as we can do it
straight away.

For wayland we need to wait till kwin is up so this is done either by
plasma-session or systemd. For systemd the unit is started explicitly so
we can parse the config file, but after the target starting kwin is
started so that the "After=" line works correctly.