Update systemd units

By marking as part of graphical-session we get a more graceful shutdown.

Also switched to explicit restart inside the unit. Originally I did this
with drop-ins, but this complicates wayland for now.

plasma-kwin_wayland.service.in

 [Unit]
 Description=KDE Window Manager
+PartOf=graphical-session.target
 
 [Service]
 ExecStart=@CMAKE_INSTALL_FULL_BINDIR@/kwin_wayland

plasma-kwin_x11.service.in

 [Unit]
 Description=KDE Window Manager
 Wants=plasma-kcminit.service
+PartOf=graphical-session.target
 
 [Service]
 ExecStart=@CMAKE_INSTALL_FULL_BINDIR@/kwin_x11 --replace
 BusName=org.kde.KWin
 Slice=session.slice
+Restart=on-failure