Fix crash forwarding to a topic from a separate window

The forward box picked a forum window for any chat of a forum that lists
its topics, including a chosen topic. When the topic's account had no
main window, it opened a new window with that id, and showing it passed
a null forum to showForum, failing the not_null assertion. The id also
never matched an already open window of that topic.

Use a forum window only for the forum itself and a chat window for a
topic, the same as the Open in new window menu item.