Move ID generation to xmpp folder

create_unique_id() was changed to use UUIDs instead of a counter in the
last commit. Since now it depends on connection_create_uuid() which is
in the xmpp subfolder the function should also be moved there.

Renamed it to connection_create_stanza_id() and moved it to
src/xmpp/connection.c.

Discussion happened in https://github.com/boothj5/profanity/pull/1010

src/xmpp/bookmark.c

@@ -333,7 +333,7 @@ _send_bookmarks(void)
 {
     xmpp_ctx_t *ctx = connection_get_ctx();
 
-    char *id = create_unique_id("bookmarks_update");
+    char *id = connection_create_stanza_id("bookmarks_update");
     xmpp_stanza_t *iq = xmpp_iq_new(ctx, STANZA_TYPE_SET, id);
     free(id);