Added term select to list of files.  #4502

This wasn't too bad so far.  I had to update the helper function that got attached vocabs to use an entity type instead of being hardcoded to node.  I also had to tell each plugin what the entity type was so the helper funciton could be called correctly.  Instead of defining this in each of their __construct's, I had them extend a parent class that set the var.  Then I went back and made each file extend it.  Not really a shortcut after all, but I think this will be useful long term and it gives me a single point of entry for documenting all the various hook methods I've created.
LOF filters by term #4502

Changed all the node specific options in the tagged node filter target entities instead.

The only real change is that the taxonomy_index table only handles tagged nodes.  I switched to og's field_data_og_vocabulary instead to get to tagged files.  There's no functional difference, but this table is a bit bigger.

Still need to test how it works with multiple vocabs.  I simplified the code a little in this case so I'm not 100% sure it's doing the same thing yet.