Fix management of actions availability

An action availability (after resolution of its filters) is set on the contributed action object => it's shared by all users.
When resolving the availability, it depends on the user and its current context, so it may interfere with other users actions.

Steps to reproduce:
1. as user A, go into any folder and select a document => "delete" action becomes available
2. as user B, go into any other folder, select a document and delete it => document is moved to trash and the "delete" action is no more available.
3. as user A, click on "delete" that is still displayed as available => nothing happens

In fact action has become unavailable because of user B action. The JSF button representing the action detects it and does not even trigger the associated action (so the method binding to seam component deleteActions is never called).

When page is displayed again after nothing has happened, action is resolved as available again => clicking again on "delete" will work as expected.

=> the service should dereference action objects when resolving availability for a given user and context.