Add initial pulseaudio module support

This seems to work quite well including hotplugging of devices, changes to
default sinks/sources etc.

So far I only provided percentage values for sink/source since they seem most
useful. Defaults are set to DEFAULT_SINK/DEFAULT_SOURCE which means it will
follow whatever defaults are at the moment (instead of being fixed to a given
device).

One thing currently left unhandled is when pulseaudio/pipewire gets
disconnected/restarted. In such a case we mark the sink/source as offline but
there is no reconnect attempted later.

Added man page + more complex example which includes mixed usage of both input
and output.

plugin.c

@@ -43,6 +43,9 @@ EXTERN_MODULE(label);
 EXTERN_MODULE(mpd);
 #endif
 EXTERN_MODULE(network);
+#if defined(PLUGIN_ENABLED_PULSE)
+EXTERN_MODULE(pulse);
+#endif
 EXTERN_MODULE(removables);
 EXTERN_MODULE(river);
 EXTERN_MODULE(sway_xkb);
@@ -123,6 +126,9 @@ init(void)
     REGISTER_CORE_MODULE(mpd, mpd);
 #endif
     REGISTER_CORE_MODULE(network, network);
+#if defined(PLUGIN_ENABLED_PULSE)
+    REGISTER_CORE_MODULE(pulse, pulse);
+#endif
     REGISTER_CORE_MODULE(removables, removables);
 #if defined(HAVE_PLUGIN_river)
     REGISTER_CORE_MODULE(river, river);