server: introduce wl_signal_emit_mutable

wl_signal_emit doesn't handle well situations where a listener removes
another listener. This can happen in practice: wlroots and Weston [1]
both have private helpers to workaround this defect.

wl_signal_emit can't be fixed without breaking the API. Instead,
introduce a new function. Callers need to make sure to always remove
listeners when they are free'd.

[1]: https://gitlab.freedesktop.org/wayland/weston/-/merge_requests/457



Signed-off-by: Simon Ser <contact@emersion.fr>
Signed-off-by: Alexandros Frantzis <alexandros.frantzis@collabora.com>