gdbus: Use symbolic constants for interfaces from dbus-specification

Most D-Bus interfaces are domain-specific, but these interfaces from the
D-Bus Specification are intended to be commonly used in any context for
which they are found to be appropriate.

Most of these use `gdbusprivate.h`. One exception is that
`gio/tests/gdbus-example-*` redefine the constants locally: due to these
files' dual role as part of the unit tests and as sample code, it seems
desirable to ensure that they can still be compiled outside GLib.

Signed-off-by: Simon McVittie <smcv@collabora.com>

gio/tests/gdbus-example-export.c

@@ -1,6 +1,8 @@
 #include <gio/gio.h>
 #include <stdlib.h>
 
+#define DBUS_INTERFACE_PROPERTIES "org.freedesktop.DBus.Properties"
+
 /* ---------------------------------------------------------------------------------------------------- */
 
 /* The object we want to export */
@@ -253,7 +255,7 @@ send_property_change (GObject         *obj,
   g_dbus_connection_emit_signal (connection,
                                  NULL,
                                  "/org/myorg/MyObject",
-                                 "org.freedesktop.DBus.Properties",
+                                 DBUS_INTERFACE_PROPERTIES,
                                  "PropertiesChanged",
                                  g_variant_new ("(sa{sv}as)",
                                                 "org.myorg.MyObject",