agent: Add support for GIO-style I/O streams

agent: Add support for GIO-style I/O streams

This adds three new classes:
 • NiceIOStream
 • NiceInputStream
 • NiceOutputStream
which allow wrapping a single stream/component pair in an I/O stream
which can be used with the standard GIO functions.

The streams are constructed as wrappers around a NiceAgent, with changes
to the NiceAgent’s properties affecting all instantiated streams.
Streams are only supported for reliable connections.

If the NiceAgent stream which underlies a NiceInputStream,
NiceOutputStream or NiceIOStream is removed, the I/O stream is marked as
closed.

This is based on work originally done by Youness Alaoui
<youness.alaoui@collabora.co.uk>.