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/private.h

@@ -59,6 +59,7 @@ GDBusProxy *           pinos_subscribe_get_proxy_finish   (PinosSubscribe *subsc
 
 typedef struct {
   guint32 version;
+  guint32 flags;
   guint32 length;
 } PinosStackHeader;