Fully embrace ALE's Message and get rid of Acceleo's IValidationMessage

[echebbi]

Closes #160

Use ALE's Message data structure to represent all validation errors. Xtext validators now return a List<Message> that is consumed by a DiagnosticsToEditorMarkerAdapter in order to create the relevant Eclipse markers.

As discussed in #160 this architecture will make easier to implement new features.

Changes

• update Xtext validators produce a list of Message instead of IValidationMessage
• enrich diagnostics.ecore metamodel with new possible errors
• read error messages from src/org/eclipse/emf/ecoretools/ale/validation/ale_editor_messages.properties (core plug-in):
  • messages are centralized and easier to update
  • messages can be translated if needed
• some error messages have been improved

Notes

• existing unit tests for Xtext validators should be updated to make sure that they produce the right Message instances. That's however a lot of work, so for now I tricked them to check the markers created by the DiagnosticsToEditorMarkerAdapter instead. That allowed to updated tests quickly to the new architecture.

[echebbi]

The code still has room for improvement (e.g. instantiation of Message instances is not very clean) but it works and allows for other issues to be solved. Let's merge and improve later.