Split monolithic Context class into pieces

[Chris Halse Rogers]

 affects pulseaudio-sharp
 status triaged
 importance medium

The Context class currently contains essentially all the bindings for
everything - sinks, sink inputs, etc. It is already annoyingly large,
and will only get larger as further context-bound objects are supported.

The Context class should instead bind only those functions that are
related to the Context itself - server event notification, connection
state, etc, and instead expose a way to grab the underlying HandleRef to
the rest of the objects. Then Sink can bind the functions relevant to
sinks, SinkInput can bind its relevant functions, etc.