Faceted search of remaining filters about photonix HOT 2 CLOSED

An example to make things clearer (photo: [tags]). Assume these are the only photos and tags in the system:

photo1: red, green, blue
photo2: red, green,
photo3: red
photo4: yellow

If the tag red is selected, the red and yellow filter gets removed from the main list of filters as photo4 can't possibly be viewed and it's the only one with the yellow tag.
If the tag green is selected, the green and yellow filters get removed from the available filters.
If the tag blue is selected, blue and yellow filters get removed.

from photonix.

Implemented and merged as of f30b3ee

from photonix.