Filter DB Extender patch

I've written quite a few patches for Drupal in the last few months, but most of them were either bugfixes or ideas/patches/tasks of others I've taken further.

Recently, I proposed the first patch, that was my own idea :

DBTNG does have the nice concept of Extenders, which are essentially a standardized Decorator pattern. You can use them to provider additional functionality to the SelectQuery class, which can build a SELECT Query with a OOP-QueryBuilder.

DBTNG currently provides two such DB Extenders, one for paging and the other for sorting. My idea was to create another one for filtering.

It basically discovers filter-able elements by exposing a hook, displays a form for them, adds simple conditions directly to the query object and also a specific tag, so that modules can do more more complex filtering based on the currently active filters.

The first version of the patch can only handle select fields, the plan is to add more of them, for example autocomplete textfields or checkboxes/radios.

Please comment on the issue if you have any questions or ideas!