Revise View_Gallery item/collection view a bit.
- add View_Gallery::collection_type().
- revise View_Gallery::init_collection() to use it to detect item collections
more directly.
- revise naming of sibling/children/collection callbacks/queries - generic
things are called "collection," and once callback is run, called "children" in
collection context and "sibling" in item context.
Add multi-tag ability ("tag/slug_1,slug_2" finds items with both tags).
Make Controller_Tags::get_tag_query() quicker for single tag queries.
Merge pull request #459 from shadlaws/fixes_20130716_1

Fixes 20130716 1