pulse: code rewrite using the asynchronous libpulse API

This is a complete rewrite of the PulseAudio output plugin.  It uses
the asynchronous API, which gives us more control over everything.
Additionally, it connects to the PulseAudio server on startup, and
keeps this connection up while MPD runs.  During pause, instead of
closing the stream, it enables "cork".

test/read_mixer.c

@@ -21,6 +21,8 @@
 #include "mixer_list.h"
 #include "filter_registry.h"
 #include "pcm_volume.h"
+#include "output/pulse_output_plugin.h"
+#include "event_pipe.h"
 
 #include <glib.h>
 
@@ -28,6 +30,17 @@
 #include <string.h>
 #include <unistd.h>
 
+void
+pulse_output_context_state_cb(G_GNUC_UNUSED struct pa_context *context,
+			      G_GNUC_UNUSED void *userdata)
+{
+}
+
+void
+event_pipe_emit(G_GNUC_UNUSED enum pipe_event event)
+{
+}
+
 const struct filter_plugin *
 filter_plugin_by_name(G_GNUC_UNUSED const char *name)
 {