Move all providers to 'lib/search/providers' in core

Additionally, this adds several get()-like functions to retrieve objects based on their ID; these are necessary for future integration with Lucene.
Format search results to HTML

Calls formatToHtml() on all search results.
Add search query to session

The last search query will be displayed by the application even if the GET 'query' parameter is not set
Add .has_icon class

All names (read <span>) with this will be indented.
Integrate 'search_lucene' into 'Advanced Search'

This represents a stable release of the 'Advanced Search' app and is ready for testing.