x11: Forward current selection state when initializing X11 selections

Most visible with xwayland-on-demand, at the time of setting things up
for X11 selections, we don't forward the current state. This makes the
first started X11 app oblivious to eg. the current clipboard.

Syncing selections up at the time of initializing the X11 selection
stuff ensures that doesn't happen.

https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1186

(cherry picked from commit 167fd07e)