Add session management

[mattcasters]

Allow all the open files Spoon to be saved in a session under a named or default session per environment, perhaps with the same name as the active environment.
When we switch to a certain environment we close all files and open the ones from the default session.
If the last session for the environment was another specific session we load that one.
Add a dialog to edit the list of transformations and jobs in the session.
Allow create/edit/delete of sessions

Store the sessions in the active metastore, not the environment metastore.

[mattcasters]

The default session is done. Now I want to allow folks to load/save named sessions.