a11y atspi: Send ChildrenChanged with correct DBus signature

The DBus XML spec for the "ChildrenChanged"
AT-SPI signal (see gtk/a11y/atspi/Event.xml)
is the following:

    <signal name="ChildrenChanged">
      <arg name="operation" type="s"/>
      <arg name="index_in_parent" type="i"/>
      <arg type="i"/>
      <arg name="child" type="v"/>
      <arg name="properties" type="a{sv}"/>
      <annotation name="org.qtproject.QtDBus.QtTypeName.In0" value="QSpiEvent"/>
    </signal>

Adhere to that when sending such a DBus signal:
Use "(siiva{sv})" instead of "(siiv@(so))"
for the GVariant type spec and pass a null
array of properties instead of sending a
reference to the sender as an argument.
(The sender is already known because it is
the event source.)

The incorrect signature could e.g. be seen
with an at-spi2-core version (git main
as of commit 422cac4bd657ce783164324e6ae4b3d54a8aa761)
with this additional local change on top:

    --- a/atspi/atspi-event-listener.c
    +++ b/atspi/atspi-event-listener.c
    @@ -28,6 +28,8 @@
     #include <string.h>
     #include <strings.h>

    +#include <stdio.h>
    +
     /**
      * AtspiEventListener:
      *
    @@ -1110,6 +1112,8 @@ _atspi_dbus_handle_event (DBusMessage *message)

       g_assert (strncmp (category, "org.a11y.atspi.Event.", 21) == 0);

    +  printf("sender: %s, signature: %s\n", sender, signature);
    +
       if (strcmp (signature, "siiv(so)") != 0 &&
           strcmp (signature, "siiva{sv}") != 0)
         {

and a corresponding pyatspi script:

    #!/usr/bin/python3
    import pyatspi
    def listener(e):
        print(f'toolkit: {e.source.get_toolkit_name()} {e.source.get_toolkit_version()}')
        print(e)
    pyatspi.Registry.registerEventListener(listener, "object:children-changed")
    pyatspi.Registry.start()

Corresponding output generated by this script without this change
in place when starting the "Entry" -> "Completion"
example from gtk4-demo:

    sender: :1.68, signature: siiv(so)
    toolkit: GTK 4.17.1
    object:children-changed:add(1, 0, [frame | Completion])
            source: [application | gtk4-demo]
            host_application: [application | gtk4-demo]
            sender: [application | gtk4-demo]

Output with this change in place:

    sender: :1.70, signature: siiva{sv}
    toolkit: GTK 4.17.1
    object:children-changed:add(1, 0, [frame | Completion])
            source: [application | gtk4-demo]
            host_application: [application | gtk4-demo]
            sender: [application | gtk4-demo]

This also matches the output seen for GTK 3's gtk3-demo:

    sender: :1.71, signature: siiva{sv}
    toolkit: gtk 3.24.43
    object:children-changed:add(1, 0, [frame | Entry Completion])
            source: [application | gtk3-demo]
            host_application: [application | gtk3-demo]
            sender: [application | gtk3-demo]

The other events (in gtk/a11y/gtkatspicontext.c) already
use the correct DBus signature.

gtk/a11y/gtkatspicontext.c

@@ -902,15 +902,13 @@ emit_children_changed (GtkAtSpiContext         *self,
       !gtk_at_spi_root_has_event_listeners (self->root))
     return;
 
-  GVariant *context_ref = gtk_at_spi_context_to_ref (self);
   GVariant *child_ref = gtk_at_spi_context_to_ref (child_context);
 
   gtk_at_spi_emit_children_changed (self->connection,
                                     self->context_path,
                                     state,
                                     idx,
-                                    child_ref,
-                                    context_ref);
+                                    child_ref);
 }
 
 static void

gtk/a11y/gtkatspiroot.c

@@ -484,8 +484,7 @@ gtk_at_spi_root_child_changed (GtkAtSpiRoot             *self,
                                     self->root_path,
                                     state,
                                     idx,
-                                    window_ref,
-                                    gtk_at_spi_root_to_ref (self));
+                                    window_ref);
 }
 
 static void

gtk/a11y/gtkatspiutils.c

@@ -339,8 +339,7 @@ gtk_at_spi_emit_children_changed (GDBusConnection         *connection,
                                   const char              *path,
                                   GtkAccessibleChildState  state,
                                   int                      idx,
-                                  GVariant                *child_ref,
-                                  GVariant                *sender_ref)
+                                  GVariant                *child_ref)
 {
   const char *change;
 
@@ -364,7 +363,7 @@ gtk_at_spi_emit_children_changed (GDBusConnection         *connection,
                                  path,
                                  "org.a11y.atspi.Event.Object",
                                  "ChildrenChanged",
-                                 g_variant_new ("(siiv@(so))", change, idx, 0, child_ref, sender_ref),
+                                 g_variant_new ("(siiva{sv})", change, idx, 0, child_ref, NULL),
                                  NULL);
 }
 

gtk/a11y/gtkatspiutilsprivate.h

@@ -36,8 +36,7 @@ gtk_at_spi_emit_children_changed (GDBusConnection         *connection,
                                   const char              *path,
                                   GtkAccessibleChildState  state,
                                   int                      idx,
-                                  GVariant                *child_ref,
-                                  GVariant                *sender_ref);
+                                  GVariant                *child_ref);
 
 void
 gtk_at_spi_translate_coordinates_to_accessible (GtkAccessible  *accessible,