GUI code must not be aware of what integers map to connection states

Fixes #9514

every linter flags this as an error

Fixes #9094

This is a MUST see https://xmpp.org/extensions/xep-0234.html

Fixes #8662

Currently, there are no enumerations using the enum module which reuse
values/need aliases. Add the @unique decorator to all enum classes as a
safety net for future modifications.

correctly close jingle session when no intersection between the remote codecs and the local ones. Fixes #8210