Rework how clients connect.

Add buffer flags. The idea is to make it possible to easily check when a
buffer contains control information that we need to parse to update the
port fields.
Make the client create remote nodes and ports and set up proxies for
them.
Make a port base class implementing most of the logic to pass buffers
locally and remotely.
Remove most code from stream.c, it's now in the port.
Make a portsink and portsrc that can write and read to/from any port. We
use these in the server to send and receive data.
Rework format negotiation. The final format is now sent in-line before
the data. The server will select a format on output ports.

pinos/client/pinos.c

@@ -104,3 +104,13 @@ pinos_fill_stream_properties (PinosProperties *properties)
 {
   g_return_if_fail (properties != NULL);
 }
+
+PinosDirection
+pinos_direction_reverse (PinosDirection direction)
+{
+  if (direction == PINOS_DIRECTION_INPUT)
+    return PINOS_DIRECTION_OUTPUT;
+  else if (direction == PINOS_DIRECTION_OUTPUT)
+    return PINOS_DIRECTION_INPUT;
+  return direction;
+}