Offer creation of the persistence descriptor after user creates a persistable class.

Currently, when user wants to use persistable classes in the process, he has to go to project settings > persistence descriptor and add the persistable classes manually there (by doing which he also creates the persistence descriptor). We don't see this process as sufficiently "out-of-box". While no other better project configuration mechanism exists, the solution could be to show user a dialog that would ask if he wants to create the persistence descriptor on the creation of the first persistable class in the project (other classes could be added to the descriptor automatically).