Make content views handle element selections

Currently the selection mechanism relies on the DocumentListManager service.

This service makes it possible to store selected documents (and documents only) and has configurability for persistence management (in session, conversation, database table).

When using several content views use the same selection list on this service, there are invalidation issues, so it could be interesting to make content views handle the cache scope.

Idea:

• make page providers handle selections: page providers already have a notion of selected elements, and sometimes we need to handle "select all" features so that could be a way to retrieve all elements directly from the page provider
• as page providers implementations are pluggable, we could keep on persisting selected documents in database tables (for the clipboard use case for instance)
• a cache of these "selection page providers" could be managed as the Seam level (like the cache of content views) so it could benefit from being in the same scope (and allow reset/refresh on Seam event behaviours)