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

gio/gio.symbols

@@ -193,6 +193,7 @@ g_cancellable_reset
 g_cancellable_cancel
 g_cancellable_connect
 g_cancellable_disconnect
+g_cancellable_source_new
 #endif
 #endif