URL detection in chat messages

[special]

Detect URLs in chat, style them, and make them clickable.

To handle clicked URLs, present the user with a choice (in a popup of some kind) between using the default browser (discouraged), launching a custom executable, or copying to the clipboard. Allow one of these options to be made the default, and provide all of them in a context menu on URLs as well.

If 'tor://' URLs are implemented for torbutton as was discussed, that could also be a worthwhile option.