wp_list_categories - Diff to make use_desc_for_title parameter more flexible

[Marventus]

Right now, the @use_desc_for_title is defined as a boolean parameter which outputs category link title attributes containing either:

• Category descriptions by default (bool 1);
  
• 'View all posts filed under [category-name]'.
  

Now, since the title attribute is not required for <a> tags in any HTML spec, I made a patch to turn this parameter into a string, with the following supported values:

• 'desc' => (default) Outputs category descriptions;
  
• 'viewall' => Same output as bool 0 (false) before: 'View all posts filed under [category-name]';
  
• 'none' => Does not output any title attributes.
  

I have tested this thoroughly, but additional testing would not hurt.

I forgot to mention that the proposed patch also introduces small changes to deprecated.php.

Enhances $use_desc_for_title in 'category-template.php' and 'deprecated.php'

It seems this had already been reported in #4596 but the fix was discarded by darkdragon. Since I already created this ticket, and the other one is very old and is closed now, I'll keep posting here.
Added the needs-codex Keyword to update the wp_list_categories Codex entry if this gets implemented.