Original commit message from CVS:
- Use the eventhandler instead of the event function to send events.
- make the scheduler setup the eventhandlers
- intercept flush events on pads links that can potentially queue data
and flush it.
- some more debugging info in spider.

I can think of one case where this flush might fail: unconnected pads where
the scheduler has not set up the eventhandler yet. I'll come up with a
solution for that soon.