Support dynamic menu contributions

[msujew]

What it does

Closes #13719

Adds a new onDidChange event to the MenuModelRegistry. Any changes to the menus (i.e. registering, unregistering, disposal) fire this event now. Adds a listener to this event in the menu factories to ensure that the menus are correctly recreated.

How to test

1. Adjust some of the sample-menu-contribution.ts file:

setTimeout(() => {
  const disposable = menus.registerMenuAction(subSubMenuPath, {
    commandId: SampleCommand2.id,
    order: '3'
  });
  setTimeout(() => disposable.dispose(), 8000);
}, 8000);

2. After starting the app, open the Sample Menu > Sample sub menu
3. It should only show 2 entries
4. After a few seconds the menu should refresh showing an additional entry
5. A few seconds later the entry should disappear again.

Follow-ups

I've noticed that the menu flashes when the model is changed (i.e. any open main sub menus are closed). That's something that could maybe be addressed in a separate PR. A smart diffing of the menu bar HTML element would probably be appropriate here.

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[msujew]

cc @kittaakos this is maybe interesting for you.