GCancellable: add g_cancellable_create_source()

g_cancellable_create_source() returns a GSource that triggers when its
corresponding GCancellable is cancelled. This can be used with
g_source_add_child_source() to add cancellability to a source.

Port gasynchelper's FDSource to use this rather than doing its own
cancellable handling, and also fix up its callback argument order to
be more normal.

https://bugzilla.gnome.org/show_bug.cgi?id=634239

docs/reference/gio/gio-sections.txt

@@ -1107,6 +1107,8 @@ g_cancellable_set_error_if_cancelled
 g_cancellable_get_fd
 g_cancellable_make_pollfd
 g_cancellable_release_fd
+g_cancellable_source_new
+GCancellableSourceFunc
 g_cancellable_get_current
 g_cancellable_pop_current
 g_cancellable_push_current