This reverts commit 28c6e0d0

.

I decided against it:

> This might actually not be such a great idea. EnsureChannel and
> CreateChannel should clearly be core, and RequestableChannelClasses
> (aka TP_CONNECTION_FEATURE_CAPABILITIES) can reasonably be core
> while connected, but Channels (and its change-notification,
> NewChannel(s) and ChannelClosed) are sufficiently special-purpose
> that I think only the AM and regression tests should be using it.
> Having the Channels and their immutable properties in the GetAll
> result seems non-optimal.

Reviewed-by: Guillaume Desmottes <guillaume.desmottes@collabora.co.uk>