Work on sink

Remove _remove from properties, we can do the same with set of a NULL
value.
Add signals to the stream API to manage the buffers. Wrap those buffers
in a GstBuffer in the pinossrc and pinossink elements and pool them in a
bufferpool.
Remove SPA_EVENT_TYPE_PULL_INPUT, we can do the same with NEED_INPUT and
by using a ringbuffer.
Do more complete allocation of buffers in the link. Use the buffer
allocator if none of the nodes can allocate.
Follow the node state to trigger negotiation and allocation.
Remove offset and size when refering to buffers, we want to always deal
with the complete buffer and use a ringbuffer for ranges or change the
offset/size in the buffer data when needed.
Serialize port_info structures as part of the port_update
Print both the enum number and the name when debuging properties or
formats.

spa/plugins/xv/xv-sink.c

@@ -452,9 +452,7 @@ spa_xv_sink_node_port_alloc_buffers (SpaNode         *node,
 static SpaResult
 spa_xv_sink_node_port_reuse_buffer (SpaNode         *node,
                                     uint32_t         port_id,
-                                    uint32_t         buffer_id,
-                                    off_t            offset,
-                                    size_t           size)
+                                    uint32_t         buffer_id)
 {
   return SPA_RESULT_NOT_IMPLEMENTED;
 }