Fix context-keys ScmProvider update, add submenu plugin contribution

[vinokurig]

Signed-off-by: Igor Vinokur ivinokur@redhat.com

What it does

Fixes #8724

• Update the ScmProvider context-keys on SCM widget start to handle properly the when condition of the related menu items.
• Apply submenu plugin contribution to be able to render submenus in the SCM view.

How to test

1. Remove "@theia/git": "^1.9.0" from examples/browser/package.json file.
2. Download vscode-git-1.49.3 and copy it to the plugins folder.
3. Open a workspace with a git project and open the SCM panel
4. Try the ... drop-down menu.
   See:
   

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

[vince-fugnitto]

I verified that the submenus present in the scm view with the builtin git extensions (both git and git-ui) are now populated 👍

[vinokurig]

@vince-fugnitto I've reworked this PR to hanle multiple nestance level of the submenus. To test it you should add such submenu contribution to the package.json of any theia plugin:

"contributes": {
    "menus": {
      "scm/title": [
        {
          "submenu": "git.custosubmenu1",
          "group": "2_main@3",
          "when": "scmProvider == git"
        },
        {
          "command": "git.clone",
          "group": "1_header@3",
          "when": "scmProvider == git"
        }
      ],
      "git.custosubmenu1": [
        {
          "submenu": "git.custosubmenu2",
          "group": "1_sync"
        },
        {
          "command": "git.sync",
          "group": "1_sync"
        }
      ],
      "git.custosubmenu2": [
        {
          "command": "git.sync",
          "group": "1_custom"
        }
      ]
    },
    "submenus": [
      {
        "id": "git.custosubmenu1",
        "label": "Custom Submenu 1"
      },
      {
        "id": "git.custosubmenu2",
        "label": "Custom Submenu 2"
      }
    ]
  }

And it will look like:

[vince-fugnitto]

@vinokurig thank you, I'll take a look at the changes shortly 👍
I believe it fixes the following issue if you'd like to track it: #8724.

[vince-fugnitto]

@vinokurig I confirmed that the pull-request works well with:

• the vscode-builtin-git extension now successfully contributing submenus
• the nested submenu example you provided Fix context-keys ScmProvider update, add submenu plugin contribution #8996 (comment)

I'm not sure if you want others to review the changes as well.