Fix wxGTK build with glib < 2.32 and streamline the code a bit

Define g_signal_handlers_disconnect_by_data() if it's not available,
i.e. when using glib older than 2.32 where it was added, to fix the
build under old systems such as CentOS 6 broken by the changes of
8278f7b618 (see #18084).

Also use it elsewhere instead of g_signal_handlers_disconnect_matched()
as it's more readable.

Closes https://github.com/wxWidgets/wxWidgets/pull/760

src/gtk/webview_webkit2.cpp

@@ -589,9 +589,8 @@ wxWebViewWebKit::~wxWebViewWebKit()
     if (m_dbusServer)
     {
         g_dbus_server_stop(m_dbusServer);
-        g_signal_handlers_disconnect_matched(
-            webkit_web_context_get_default(), G_SIGNAL_MATCH_DATA,
-            0, 0, NULL, NULL, m_dbusServer);
+        g_signal_handlers_disconnect_by_data(
+            webkit_web_context_get_default(), m_dbusServer);
     }
     g_clear_object(&m_dbusServer);
     g_clear_object(&m_extension);