[Ticket/11936] Notification Avatars - Fixes and Restyling

1. Fix an issue where Avatars overflow into the next list element if the text next to the avatar is 2 lines or less
2. Remove underline/decoration from the text in the notification window
3. Use phpBB's no_avatar.gif for users with no avatars in the notification window.
4. New images for Notification and PM icons and No Avatar (all new images are created from scratch, no licensing concerns)

http://tracker.phpbb.com/browse/PHPBB3-11936
http://area51.phpbb.com/phpBB/viewtopic.php?f=108&t=44779