Fixed a bug: exception when no id list was loaded.
Relace the new session dialog by a wizard and multiple id lists.

- Now the new session option opens a wizard with two pages (first page:
directory and exam config file; second page: id list files).

- Several id files can be chosen (before it was only one).

- The id files are now stored inside the session directory.

- It is not possible now to create a session in a non-empty directory.