xsettings: Add support for XFixes ClientDisconnectMode
The Xserver itself is capable of terminating itself once all X11 clients are gone, yet in a typical full session, there are a number of X11 clients such as gsd-xsettings running continuously.
Those always-running clients will prevent the Xserver from terminating, because the actual number of X11 clients will never drop to 0.
Use XFixes ClientDisconnectMode to inform the X11 server that it can terminate even if ibus-x11 is still running.
That will allow Xwayland from terminating automatically when regular clients have quit.
On plain Xorg servers, the lifetime of the session is usually tied to the session manager or window manager, and this change will have no effect.