Fix user names not being displayed in group chats

Add users to temporary group "Telegram Foreign", to assure that the user
alias is always known to Pidgin. Don't store this group permanently but
drop it when closing.

tgp-2prpl.c

@@ -116,6 +116,22 @@ void p2tgl_got_chat_left (struct tgl_state *TLS, tgl_peer_id_t chat) {
 void p2tgl_got_chat_in (struct tgl_state *TLS, tgl_peer_id_t chat, tgl_peer_id_t who,
                         const char *message, int flags, time_t when) {
   char *name = p2tgl_strdup_id (who);
+
+  // Make sure that the participant exists in the blist, so that the correct print name of
+  // the chat is being displayed by libpurple
+  connection_data *conn = TLS->ev_base;
+  PurpleBuddy *buddy = buddy = purple_find_buddy (conn->pa, name);
+  if (! buddy) {
+    buddy = purple_buddy_new (conn->pa, name, tgl_peer_get(TLS, who)->print_name);
+    PurpleGroup *group = purple_find_group ("Telegram Foreign");
+    if (!group) {
+      group = purple_group_new ("Telegram Foreign");
+      purple_blist_add_group (group, NULL);
+      purple_blist_node_set_flags (&group->node, PURPLE_BLIST_NODE_FLAG_NO_SAVE);
+    }
+    purple_blist_add_buddy (buddy, NULL, group, NULL);
+    purple_blist_node_set_flags (&buddy->node, PURPLE_BLIST_NODE_FLAG_NO_SAVE);
+  }
 
   serv_got_chat_in (tg_get_conn(TLS), tgl_get_peer_id (chat), name, flags, message, when);