Remove unused Item::move().  To move an item, just change its parent_id and let
Model_Item and the item_moved event do the dirty work.  The only place I found
that references this function is the defunkt gwtorganize contrib module, so it
seems safe to go.
Rename Search::search_query_base() --> Search::get_search_query().
Rename Item::set_display_context_callback() --> Item::set_display_context() and
Item::clear_display_context_callback() --> Item::clear_display_context(), which
match Item::get_display_context().
Add @todo notes to User and Group classes - not sure if these errors still exist
Big rewrite of item/collection views (display context, siblings,
position, breadcrumbs, pagination, etc.)
- strip down Item::get_display_context() to just get context from cache.
- revise View_Gallery::init_collection() to use callbacks for the collection
query and breadcrumbs and to set the *same* callbacks as the item display
context.  This simplifies a lot of things...
- add View_Gallery::init_item(), analogous to View_Gallery::init_collection(),
which uses the display context and sets all sibling/position/pagination info.
- revise View_Theme::siblings() to match.
- strip down Breadcrumb::array_from_item_parents().
- add Breadcrumb::set_first_and_last().
- revise item collections (album, tag, search) to use new setup.  Among other
things, this removes a lot of position/sibling/display_context functions.
- revise non-item collections (users, comments) to use the new setup.
- revise the search breadcrumbs to include the "show" parameter as well as
have a more descriptive title (i.e. make it similar to the tag breadcrumbs).
Merge pull request #454 from shadlaws/fixes_20130711

Fixes 20130711