[feature/avatars] Use "Main" as category for avatars in root of gallery

Before this change the whole avatar gallery path would show as category.
Additionally, the avatars that were selected like that had an incorrect
path and didn't show up correctly.
With this patch it'll display "Main" as category and properly work.

PHPBBB3-10018

phpBB/language/en/common.php

@@ -358,6 +358,7 @@ $lang = array_merge($lang, array(
 	'LOGOUT_USER'						=> 'Logout [ %s ]',
 	'LOG_ME_IN'							=> 'Remember me',
 
+	'MAIN'					=> 'Main',
 	'MARK'					=> 'Mark',
 	'MARK_ALL'				=> 'Mark all',
 	'MARK_FORUMS_READ'		=> 'Mark forums read',