Allow to exclude arguments from being populated by facet blocks

Summary
The attached patch adds a setting to arguments that allows arguments to be excluded from being populated by facet blocks. This patch also cleans up the URL generation code in apachesolr_views_query.inc and thereby properly fixes #856522: Taxonomy facet blocks no longer working without having to special case taxonomy arguments.

Details
Currently, an argument always gets populated if a facet for the same field as an argument is available. For example, this makes it impossible to use taxonomy facets if also relying on a taxonomy tid passed from the URL (as on taxonomy/term/%tid). Currently, when on taxonomy/term/1, a taxonomy facet (no matter whether in the same vocab or not) has a link like taxonomy/term/1,66.

With this patch, instead, you can just add two taxonomy term arguments. You then set the first to not be populated by facet blocks, while leaving the second at the default (i.e., allow to be populated by facet blocks). You then get nice links like taxonomy/term/1/66. You can set the second argument to accept a list of tids and simply add as many taxonomy facet blocks to your nice apachesolr powered taxonomy listing without breaking the basic View URL.

This patch also switches to consistently using the array keys of $this->_facets instead of apachesolr_get_enabled_facets(), which works for taxonomy terms without having to special case anything.

We're using this patch successfully to power all taxonomy term listings with apachesolr_views and taxonomy term facets (for different vocabs) without "losing" the term actually being viewed.

This patch is sponsored by Xarxa Media GmbH.