Consider adding a filter by collection when searching Tags

[patrickmj]

From this question on the forums, it looks like a reasonable use case to make the items/tag page be able to filter by collection.

Seems like it'd be just a small addition/rework to these lines in the Tag table to use the Item table's existing filters if a collection or collection_id param is set.

            //If we only want tags for public items, use one of the ItemTable's filters
            if ($params['type'] == 'Item' && isset($params['public'])) {
                $db->getTable('Item')->filterByPublic($select, (bool) $params['public']);
            }