Add g_source_set_dispose_function() for setting a dispose function for GSource

This allows GSource implementors to safely clear any other references to
the GSource while the GSource is still valid, unlike when doing the same
from the finalize function.

After the dispose function has run, it is valid for the reference count
of the GSource to be > 0 again to allow the case where another thread in
the mean-time got access to the GSource reference before the dispose
function was called.

This allows fixing a thread-safety issue in the GCancellable, GstBus and
various other GSource implementations.

glib/gmain.h

@@ -204,6 +204,20 @@ typedef gboolean (*GSourceFunc)       (gpointer user_data);
 typedef void     (*GChildWatchFunc)   (GPid     pid,
                                        gint     status,
                                        gpointer user_data);
+
+
+/**
+ * GSourceDisposeFunc:
+ * @source: #GSource that is currently being disposed
+ *
+ * Dispose function for @source. See g_source_set_dispose_function() for
+ * details.
+ *
+ * Since: 2.64
+ */
+GLIB_AVAILABLE_TYPE_IN_2_64
+typedef void (*GSourceDisposeFunc)       (GSource *source);
+
 struct _GSource
 {
   /*< private >*/
@@ -536,6 +550,13 @@ GMainContext *g_main_loop_get_context (GMainLoop    *loop);
 GLIB_AVAILABLE_IN_ALL
 GSource *g_source_new             (GSourceFuncs   *source_funcs,
                                    guint           struct_size);
+
+G_GNUC_BEGIN_IGNORE_DEPRECATIONS
+GLIB_AVAILABLE_IN_2_64
+void     g_source_set_dispose_function (GSource            *source,
+                                        GSourceDisposeFunc  dispose);
+G_GNUC_END_IGNORE_DEPRECATIONS
+
 GLIB_AVAILABLE_IN_ALL
 GSource *g_source_ref             (GSource        *source);
 GLIB_AVAILABLE_IN_ALL