core: Added new hooks: PA_CORE_HOOK_SOURCE_PORT_CHANGED and PA_CORE_HOOK_SINK_PORT_CHANGED

This allows modules to know when certain ports are changed.
This will allow e.g. a filter module (or LADSAP) to only load
when a certain port is used on the device (e.g. to only filter
headphones and not normal speakers).

(Comment from Colin Guthrie: This may also have use in UCM)

src/pulsecore/source.c

@@ -1571,5 +1571,7 @@ int pa_source_set_port(pa_source *s, const char *name, pa_bool_t save) {
     s->active_port = port;
     s->save_port = save;
 
+    pa_hook_fire(&s->core->hooks[PA_CORE_HOOK_SOURCE_PORT_CHANGED], s);
+
     return 0;
 }