[API] Provide a way to add update the environment

[echebbi]

Motivation

Sometimes the ALE environment must be updated programmatically with new ALE source files or Ecore models (e.g. for a "New ALE file" wizard or a "add file to environment" quick fix). However the IAleEnvironment currently offers no way to do so and we have to check manually whether the environment is based on preferences or on a .dsl file then make changes accordingly.

That's not scalable.

Proposed Solution

Update IAleEnvironment with methods to:

• add a new Ecore model,
• add a new ALE source file,
• persist the changes.