GSimpleAsyncResult: support reliable cancellation

Add a function g_simple_async_result_set_check_cancellable() to provide
a GCancellable that is checked for being cancelled during the call to
g_simple_async_result_propagate_error().

This gives asynchronous operation implementations an easy way to
provide reliable cancellation of those operations -- even in the case
that a positive result has occured and is pending dispatch at the time
the operation is cancelled.

https://bugzilla.gnome.org/show_bug.cgi?id=672013

gio/gsimpleasyncresult.h

@@ -83,6 +83,8 @@ gboolean            g_simple_async_result_get_op_res_gboolean (GSimpleAsyncResul
 
 
 
+void                g_simple_async_result_set_check_cancellable (GSimpleAsyncResult *simple,
+                                                                 GCancellable       *check_cancellable);
 gpointer            g_simple_async_result_get_source_tag   (GSimpleAsyncResult      *simple);
 void                g_simple_async_result_set_handle_cancellation (GSimpleAsyncResult      *simple,
 								   gboolean          handle_cancellation);