Spec for transferring VoIP Calls

Both sides will have to support this for it to work so it's going to suck if we get clients that don't support it: we should spec it sooner rather than later.

Requirements:
- Between different devices (same person)
- Same device moving networks
- Transferring a call switchboard style in-house
- Possibly: just redirecting a call somewhere else entirely?

Some of these would be just transferring the media (ie. ICE renegotiation). Others would involve moving signalling too.

For just moving media, it would be nice to keep this as simple as possible, possibly just telling the client to start ICE over again.

(Imported from https://matrix.org/jira/browse/SPEC-150)

(Reported by @dbkr)

No files, tests, or entry points are named. Start by reviewing the listed transfer cases and the distinction between media transfer and signalling transfer, then determine the protocol scope and client compatibility requirements. Done means an agreed specification covers the required scenarios and clearly defines what each client must support.

Written by the indexing model from the issue text.