online_editor: Code navigation between files

[hunger]

Enable code navigation between open files.

E.g. go to definition for "Homepage" in the printer demo's main page does work now. It used to open a new tab with the source code in it before.

[tronical]

I think url would be a more readable name :). Maybe set_model could also be renamed to set_current_url since it does take a url as a parameter and only internally uses the models.

Suggested change

	{ resource, options },
	source: monaco.editor.ICodeEditor | null,
	_sideBySide?: boolean,
	): Promise<monaco.editor.ICodeEditor | null> => {
	if (editor == null) {
	return Promise.resolve(editor);
	}
	if (!this.set_model(resource.toString())) {
	return Promise.resolve(null);
	{ resource: url, options },
	source: monaco.editor.ICodeEditor | null,
	_sideBySide?: boolean,
	): Promise<monaco.editor.ICodeEditor | null> => {
	if (editor == null) {
	return Promise.resolve(editor);
	}
	if (!this.set_model(url.toString())) {
	return Promise.resolve(null);

[hunger]

Yes, that should change, but not as part of this patch: There are several methods that have Model in their name and they all should change.

I also want to use Url directly over converting that to and from string in several places.