local-display-factory: Provide more flexibility for configuring display server
There's currently a way to disable wayland, but no way to disable Xorg. We currently prefer wayland if it's not disabled, but have no way to prefer Xorg without disabling wayland entirely.
There's currently no way use legacy Xorg support at all if user display server support is enabled at a build time.
This commit adds more flexibility to display server selection. It adds two new keys: XorgEnable and and PreferredDisplayServer.
XorgEnable=false disables Xorg support entirely on seat 0.
PreferredDisplayServer can be set to "wayland", "xorg", "legacy-xorg" or "none" to select which display server is used by default. If it's set to "wayland", it will fall back to "xorg". If it's set to "xorg" it will fall back to "wayland".