Add hyperlink support to GUI engine

[historic_bruno]

Show a formatted, clickable link with optional description. I could see it being defined in the BB-style tags we currently use, as follows:

"Visit the [url=http://www.wildfiregames.com/forum/index.php?act=idx]Wildfire Games forums[/url] for more information"

This would be useful any time we wanted to have external links in the game without requiring a separate button (that's not always feasible anyway).