Move X11 specific code from Display to DisplayServer

A step forward for a display server agnostic sddm.
With Wayland support coming soon we want display server specific code
not scattered all over the place.
Set HAVE_SYSTEMD when building with systemd support
Log display server command line

For debugging purpose.
Turn DisplayServer into an abstract class

Move Xorg specific code to XorgDisplayServer, this will allow us to
support multiple display server.
Update change log
One CMakeLists.txt file for each subdirectory

It's easier to work with separate CMakeLists.txt files, one for each
subdirectory.