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