basesink: clean up event handling

basesink: clean up event handling

Add new wait_eos vmethod to wait for the eos timeout before posting the EOS
message on the bus.
Add default event handler. Move the default event actions in there. Call the
event vmethod from the pad event handler. Subclasses are now supposed to chain
up to the parent event handler or unref the event and do their own thing.
Avoid passing unused parameters to functions.