Make parsedquery customizable

This pull request is quite complex, so before looking at the code there's something to explain.

First of all: the actual plone.app.querystring seems a lot complex if compared to old collections.
The idea that forced me to branch is about creating a new collection criteria that is not strictly related to an existing catalog index. This is totally unsupported right now by collections and p.a.querystring.

So: this branch will make possible to define one or more "parsed query index modifiers" by defining named utilities.
Those utilities can:
* modify the query
* modify the used index

Apart my final motivation (look at https://github.com/keul/collective.typecriterion) this change make possible to translate the fix for Subject index that now live in p.a.querystring code (that is quite ugly) to something more clean (the Subject index modifier is part of the branch).