Add staging system bell protocol

This is meant to let applications ring the system bell. It needs to be a
Wayland protocol because a system bell is not necessarily audiable; for
for example accessibility reasons, it might need be a visual feedback,
which may be tied to a specific window. Accessibility features are
usually configured globally, and one likely wants identical visual
feedback for all system bell ringings, so it doesn't fit as a client
side only feature.

This aims to replaced and deprecate the `gtk_shell1.system_bell`
request.

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>

meson.build

@@ -57,6 +57,7 @@ staging_protocols = {
 	'tearing-control': ['v1'],
 	'xdg-activation': ['v1'],
 	'xdg-dialog': ['v1'],
+	'xdg-system-bell': ['v1'],
 	'xdg-toplevel-drag': ['v1'],
 	'xdg-toplevel-icon': ['v1'],
 	'xwayland-shell': ['v1'],

staging/xdg-system-bell/README

+system_bell protocol
+
+Maintainers:
+Jonas Ådahl <jadahl@gmail.com>
+Carlos Garnacho <carlosg@gnome.org>

staging/xdg-system-bell/xdg-system-bell-v1.xml

+<?xml version="1.0" encoding="UTF-8"?>
+<protocol name="xdg_system_bell_v1">
+  <copyright>
+    Copyright © 2016, 2023 Red Hat
+
+    Permission is hereby granted, free of charge, to any person obtaining a
+    copy of this software and associated documentation files (the "Software"),
+    to deal in the Software without restriction, including without limitation
+    the rights to use, copy, modify, merge, publish, distribute, sublicense,
+    and/or sell copies of the Software, and to permit persons to whom the
+    Software is furnished to do so, subject to the following conditions:
+
+    The above copyright notice and this permission notice (including the next
+    paragraph) shall be included in all copies or substantial portions of the
+    Software.
+
+    THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+    IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+    FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
+    THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+    LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+    FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+    DEALINGS IN THE SOFTWARE.
+  </copyright>
+
+  <interface name="xdg_system_bell_v1" version="1">
+    <description summary="system bell">
+      This global interface enables clients to ring the system bell.
+
+      Warning! The protocol described in this file is currently in the testing
+      phase. Backward compatible changes may be added together with the
+      corresponding interface version bump. Backward incompatible changes can
+      only be done by creating a new major version of the extension.
+    </description>
+
+    <request name="destroy" type="destructor">
+      <description summary="destroy the system bell object">
+	Notify that the object will no longer be used.
+      </description>
+    </request>
+
+    <request name="ring">
+      <description summary="ring the system bell">
+	This requests rings the system bell on behalf of a client. How ringing
+	the bell is implemented is up to the compositor. It may be an audible
+	sound, a visual feedback of some kind, or any other thing including
+	nothing.
+
+        The passed surface should correspond to a toplevel like surface role,
+        or be null, meaning the client doesn't have a particular toplevel it
+        wants to associate the bell ringing with. See the xdg-shell protocol
+        extension for a toplevel like surface role.
+      </description>
+      <arg name="surface" type="object" interface="wl_surface"
+	   allow-null="true" summary="associated surface"/>
+    </request>
+  </interface>
+</protocol>