Need a per-subscription callback context parameter

Suppose you have an application with an unspecified number of windows, which need e.g. drag gestures. Currently you can only have one function (existing at compile time) that as a callback. Thus the application can not tell which window should receive which gesture unless they maintain their own window id -> window object mapping table. This is cumbersome and extra work for developers.

An alternative is that the user can specify a per-subscription context (void pointer) that they can then extract from the event object, just like the callback parameters in Gtk+ and Qt.