windowManager: X11 can work without gsd-xsettings

Currently, a failure to start the Systemd unit gsd-xsettings.target would be considered a failure to start Xwayland.

That means that if gsd-xsettings.target fails to start for whatever reason, no X11 client can be used on Wayland.

However, XSettings is by no mean mandatory for X11 clients and many legacy X11 clients do not implement XSettings. Those who do always have a fallback path and therefore can still work without XSettings.

Make a failure to start the Systemd unit gsd-xsettings.target non blocking for Xwayland, and just log a warning message.