Support in the context API for connecting to a Pinos instance
listening at a specified socket - not only to the one pointed out by
the environment variables XDG_RUNTIME_DIR and PINOS_CORE.
Move send and dispatch functions to the implementation. This makes it
possible to place an access check before sending and dispatching.
Add module-access that allows to bind and notify on globals owned by
the client.
Remove the client object, it is not very useful now that we have the
nodes.
Fix some properties on the proxy objects.
Use sendmsg and recvmsg directly because the GIO ones do allocations.
make pinos_properties_merge and use it to combine properties from nodes
and ports.
