Support for /etc/xdg/Xwayland-session.d setup scripts

[wengxt]

GNOME is making use of this directory to setup required X11 service whenever Xwayland is started on demand.

https://github.com/GNOME/gnome-settings-daemon/blob/6a4e36baba5815423c61c8b21c225055054905e4/plugins/xsettings/gsd-xsettings-manager.c#L1236

This is required to properly support, but not limited to
Using input method under Xwayland (ibus / fcitx)
pulseaudio
at-spi

As those X11 client will mark themself as "non-blocking" to allow Xwayland to quit even if they are still connected to Xwayland. (a.k.a XCB_XFIXES_CLIENT_DISCONNECT_FLAGS_TERMINATE)