From #24936:

Some comments while working on the Call example CM:

* InitialTransport should be specified to be "" where not applicable, and in
particular, on CMs with hardware streaming

* The immutable properties on Channel, Stream and Content should all be
annotated as such

More thoughts:

Sjoerd and I agreed that the RequestableChannelClasses should be:

* fixed = { Call, CONTACT, InitialAudio=TRUE }, allowed = { InitialVideo=TRUE }
* fixed = { Call, CONTACT, InitialVideo=TRUE }, allowed = { InitialAudio=TRUE }

i.e. clients aren't allowed to make outgoing calls that have neither initial
audio nor initial video. This doesn't match StreamedMedia, and doesn't match
telepathy-qt4's current handling of StreamedMedia capabilities - it'll be
important to check at review that it gets this right.

The initial streams should have the Initial disposition, even though that's not
interesting on locally-initiated streams.