Convert filter links into standard form buttons

With this change, the filter box links shows as follows:

![screen shot 2014-11-23 at 12 59 41 pm](https://cloud.githubusercontent.com/assets/1354889/5159208/bd17d136-7310-11e4-89e1-46d542397029.png)

Fixes #17834
I realize this is not directly related to the change submitted for review, but as I was reviewing the code I started wondering what the `'6'` magic constant was all about. I think we should define it, as well as other values for *type* (and possibly `'simple'` and `'advanced'` also) in constants_inc.php.  

Thoughts ?