CamelStream: Add a "base-stream" property.

CamelStream: Add a "base-stream" property.

CamelStream is no longer an abstract class.  It can now act as a thin
GIOStream wrapper by way of camel_stream_new().

This is a step towards elminating CamelStream itself.  The plan is to
start removing CamelStream subclasses and using GIO streams directly,
until only CamelStream remains.  Then replace CamelStream arguments
with GIOStream/GInputStream/GOutputStream in higher level APIs like
CamelDataWrapper.