Allow overrides.json to override default shortcuts.

[jasongrout]

References

Fixes https://discourse.jupyter.org/t/keyboard-shortcuts-with-overrides-json/5751 and https://discourse.jupyter.org/t/some-configs-in-overrides-json-does-not-work-as-i-intended/3955

Code changes

Shortcuts were throwing away any defaults given by the shortcut plugin settings or overrides.json. This adds them back into the default shortcuts, and also allows default shortcuts to disable each other silently. This allows overrides.json to disable default shortcuts as well as add new default shortcuts.

User-facing changes

Users can override default shortcuts with overrides.json.

This behavior is slightly different than the normal overrides.json behavior. Normally, the overrides.json values overwrite the plugin defaults, so the user never sees the plugin defaults. In this PR, we instead add the overrides.json shortcuts, merging them into the existing defaults, rather than overwriting the existing defaults. I think this makes more sense for shortcuts, which are assembled from many different plugins.

To test, you can create a dev_mode/settings/overrides.json file like this:

{
  "@jupyterlab/apputils-extension:themes": {
      "theme": "JupyterLab Dark"
  },
  "@jupyterlab/shortcuts-extension:shortcuts": {
      "shortcuts": [
      {
          "command": "imageviewer:flip-horizontal",
          "keys": [
              "X"
          ],
          "selector": ".jp-ImageViewer"
      },
      {
        "command": "imageviewer:flip-horizontal",
        "keys": [
            "H"
        ],
        "selector": ".jp-ImageViewer",
        "disabled": true
    }
    ]
  }
}

You can know it is active by the default dark theme. Look at the shortcuts, and you'll see the two additional shortcuts, and then try the shortcuts in the imageviewer to verify that "X" now flips horizontally and "H" does not.

Backwards-incompatible changes

[jupyterlab-dev-mode]

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[meeseeksmachine]

This pull request has been mentioned on Jupyter Community Forum. There might be relevant details there:

https://discourse.jupyter.org/t/keyboard-shortcuts-with-overrides-json/5751/3

[meeseeksmachine]

This pull request has been mentioned on Jupyter Community Forum. There might be relevant details there:

https://discourse.jupyter.org/t/some-configs-in-overrides-json-does-not-work-as-i-intended/3955/3

[blink1073]

Thanks! Needs a lint pass but otherwise LGTM.

[jasongrout]

It adds the shortcut, just as if the user had given it in the user preferences. If you want to disable an existing shortcut, use the disabled: true field, just like you would in the user preferences.