Add viewtopic_topic_title_after template event

There are two events in viewtopic that allow extensions to hook into the topic title (for purposes of prefixes, for example). However, there is no event for adding something AFTER the topic title. viewtopic_topic_title_append forces the extension to take on the h2 header. For extensions like the topic descriptions extension I'm writing, it's desirable to have an event after the closing </h2> tag. I'm therefore proposing an event titled viewtopic_topic_title_after.