Allow shortcuts to be empty and to have win/mac/linux-specific keys.

[jasongrout]

References

See phosphorjs/phosphor#438 for some discussion.

Code changes

Phosphor allows shortcuts to be empty, and it’s the only way to have platform-specific shortcuts (define keys to be [], and the platform keys to be something specific). We didn't allow this, but we should.

User-facing changes

Backwards-incompatible changes

None

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[blink1073]

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[jasongrout]

Looks like there a few other places that have to change in the code to actually get this to work.

[jasongrout]

In particular, having an empty list in the keys field triggers this error that skips the shortcut:

jupyterlab/packages/coreutils/src/settingregistry.ts

Lines 886 to 889 in 70ffebf

	if (!keys) {
	console.warn('Shortcut skipped because `keys` are [""].', shortcut);
	return false;
	}

So that needs to be fixed as well.

[jasongrout]

#7589 supersedes this.