Only usable with the patches from
https://bugzilla.gnome.org/show_bug.cgi?id=697907

SPECIFIC: agent: Fix GSource dispatches for UDP NiceOutputStreams

The GSource created by g_pollable_output_stream_create_source() on a
NiceOutputStream would never have been dispatched, due to not having any
poll events (or anything else); it only works for TCP streams because of
a GCancellable child source which is triggered by the TCP code.

Fix that by making ComponentSource suitable for G_IO_IN and G_IO_OUT
again, and adding that as a child source for UDP output stream GSources.

agent: SPECIFIC: Drop support for reliable streams

libnice should not be implementing pseudo-TCP support internally — it
should provide the PseudoTcpSocket as a separate API which user code can
use if it needs. If this were to be upstreamed, a deprecation path would
have to be written, continuing to support the old API. However, that
would take too long, so this commit just rips it all out.

This commit *must not* be upstreamed.

https://phabricator.collabora.co.uk/T447

agent: Add nice_agent_get_socket() to return a GDatagramBased

TODO: Needs documentation and testing.

This bumps the GLib dependency to 2.46 for GDatagramBased.

agent: Remove nice_agent_get_io_stream()

It has been replaced by nice_agent_get_socket().

TODO: This needs documenting and testing.

TODO: This should *not* go upstream without turning it into a
deprecation-fest.