[Telepathy] Spec updates for tubes channel

[Alban Crequy]

I started to work on how tube channels can be requested with the new
requestotron. Channel type Channel.Type.Tube will replace
Channel.Type.Tubes in order to have only 1 tube per channel.

There is 2 steps to establish a tube:

1/ request the channel with
Connection.Interface.Requests.DRAFT.RequestChannels

2/ offer the tube in the channel with
Channel.Type.Tube.OfferTube

A tube channel can have theses properties:

- TubeType - s ("dbus" or "stream")
- ServiceName - s ("vnc")
- Parameters - a{sv}
and in case of stream tubes:
- AddressType - u (stream tubes only)
- AccessControl - v
- AccessControlParam - u

Each of theses properties can be setup by either the D-Bus method call
RequestChannels (in step 1) or OfferTube (in step 2). The 2 D-Bus
methods can be called from 2 different applications. I don't have a real
use-case in mind but the address book use-case (req2) suggests it.
Maybe start a game from an address book?

ServiceName needs to be setup by RequestChannels in order to be
able to dispatch the channel locally. AddressType, AccessControl and
AccessControlParam are specific to stream tubes and need to be chosen
by the application which offers the service, so they must be setup by
OfferTube.

I am not so sure about TubeType and Parameters. Does the address
book need to know that the tic-tac-toe game uses stream tubes and
not D-Bus tubes?

-- 
Alban