[Jingle] Fix misplaced tags in error stanzas ; added error type support (for types other than cancel)

Fixes #5668, #5651

holy diff batman!

[Jingle] Volume levels are saved, audio UI shows up even if there are errors, until the session ends

There is still room for improvement, but it should be better.

This allows jingle availability to be updated if contact sign in/out.
This patch will also wait for user acceptance before connecting.
This will, among other things, ensure that audio/video state won't be set to
JINGLE_STATE_CONNECTING while the connection is already up.

A content has now to be accepted (user-side) before {content,session}-accept

new GUI to start audio and video sessions. TODO: ability to add/remove audio/video content to an existing session