wlheadless: Add support for kwin
Even though kwin_wayland
supports passing a command, it seems the actual
arguments following the command get ignored, so this is not suitable for
wlheadless.
Instead, we spawn kwin_wayland
and the clients separately like we do for
weston.
This seems to work better than the approach taken in !4 (closed)
/cc @Conan_Kudo