Add API support for submenus

[egamma]

Add support for submenus to simplify the menu structure of the Git extensions '...' menu.

[gjsjohnmurray]

See #9827

I'd particularly like #9827 (comment) to be catered for in the new API, so an extension can contribute a submenu whose contents are computed at display-time.

[joaomoreno]

Syntax proposal:

{
  "contributes": {
    "menus": {
      "scm/title": [
        {
          "command": "git.commit",
          "group": "navigation"
        },
        {
          "submenu": "git.stage",
          "group": "navigation"
        }
      ],
      "git.stage": [
        {
          "command": "git.stageAll",
          "group": "navigation"
        },
        {
          "command": "git.stageUntracked",
          "group": "navigation"
        }
      ]
    },
    "submenus": [
      {
        "id": "git.stage",
        "label": "Stage",
        "icon": "$(something)"
      }
    ]
  }
}

[gjsjohnmurray]

Syntax proposal:

@joaomoreno Maybe extend the syntax of submenus contribution to support submenus that call the extension at display-time to obtain their contents.

[joaomoreno]

That will definitely not be in there for the first iteration. That should even be generalized for regular menus.

[joaomoreno]

@jrieken Added the icon property in the submenus contribution, in order to make submenus work in toolbars. 👍

[jrieken]

Re-opening for the finalisation process...

[joaomoreno]

@jrieken I had this one created: #103946

I leave it up to you to decide which one to keep. I won't be able to work in finalizing it since I'm out in August so after choosing the issue, pick one option:

1. Keep it in August and assign someone else;
2. Move it to September and assign myself.

[jrieken]

I leave it up to you to decide which one to keep.

The newish flow is to have a single item that can be followed throughout the phases of idea, proposal, and finalization. September is good for me

[yubaoquan]

When will submenu apis be stable?

[joaomoreno]

With the next upcoming stable release: September 2020.

[joaomoreno]

Verification

This new API should be usable without the proposed flag!

[erythana]

Is it possible to change the Submenu via Code? (= alter the Package.json?)
I need to expose some menuitems programatically..

Pinging @joaomoreno
Sorry for pinging you - do you know whether this is possible or will be available in the near future?