GLSP-1531: Introduce viewport change event

[tortmayr]

What it does

Refactor our approach to handle model changes and introduce dedicated IModelChangeService.

• Extends the ICommandStack interface by adding an event to react to successfull command execution
• Introduce dedicated IModelChangeService to encapsulate model change handling from the EditorContextService similar to how we do it for the selection change and the SelectionService
• Refactor editor context, to use/forward to the model change service

In addition:

• Clean up disposal behavior of FokusTrack

Fixes eclipse-glsp/glsp/issues/1531

How to test

For testing a logger can be configured as event listener e.g:

Follow-ups

Changelog

• This PR should be mentioned in the changelog
• This PR introduces a breaking change (if yes, provide more details below for the changelog and the migration guide)
  Potentially breaking changes for Adopters that extend internal API:
• GLSPCommandStack: 'notifiyListener' method has been removed
• EditorContextSerivce:
  • Remove _modelRoot and onModelRootChangedEmitter properties. Corresponding getters now forward to the ModelChangeService
  • Remove notifyModelRootChanged method (Probably no side effect for adopters. Method was only callable from commandstack anyways)

[tortmayr]

@martin-fleck-at After our offline discussion, I have now pushed a new Proposal and updated the PR description

[martin-fleck-at]

Overall very good change, I just have a few comments and questions :-)

[martin-fleck-at]

LGTM!