This is done from the backend X11 connection, but needs directing at times
from the frontend X11 connection. Commit 5a8509f89503 added a XEvent
argument presumably for possible future expansions that did never come.
Since this function is nothing about events, drop the XEvent argument and
make the name a little bit more ad-hoc (according to what it does, at
least).
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3153>