audio encoder/decoder base classes needlessly delay caps events

Submitted by Sebastian Dröge @slomo

Link to original bug (#733148)

Description

+++ This bug was initially created as a clone of Bug 733147 +++

The encoder base classes delay caps events currently. This was added as part of bug 680614.

Reason is that a few encoders need to wait until they get other sticky events (tags!) before they can negotiate... as the tags are part of the caps (vorbis streamheaders, etc).
However doing this delaying unconditionally for everything in the base class seems wrong and causes problems (Nicolas has details). If anything the subclasses have to delay, as only the subclasses really know how long to delay. Currently we always delay until the first buffer but that might be too late or too early!

I would like to change this for 1.6, even if it is an ABI change. To make that possible we will have to fix all subclasses though.