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.

[rschnekenbu]

@msujew, on my machine (Linux), I tried both electron and browser version (also both on original branch and branch rebased on latest master).
On browser version, the menu disappears when it is refreshing. If I open it again, I have the right number of commands depending on the timeout. I understand the menu should refresh and content is added/remove, but it should stay visible?
On electron version, the menu stays (there is a kind of small flash on the top menu, as it was deselected), but the content is not updated. If I close it and reopen it, then the content is correct.
I can of course run some screen recording to show you what is happening on my machine for both version if you want.

[msujew]

@rschnekenbu I've mentioned this in the follow-ups of the PR:

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.

The main issue is that on the browser, the whole HTML element is replaced, which in turn leads to losing some state information. I would try looking into this in a separate PR as indicated above.

In Electron, we're bound by the limits of the underlying OS. I'm not even sure you can add/remove native window menu entries while they're open on Windows (my OS) or other OS'. I'll take a look at this as well, but I can't promise anything here.

[rschnekenbu]

@rschnekenbu I've mentioned this in the follow-ups of the PR:

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.
Indeed, that makes sense. This could be indeed part of a second PR.

Other than these rendering issues, I am OK with this PR. I will approve it.

[rschnekenbu]

Looks good with of course the current known limitations :) Thanks @msujew!