jsdoc: core>Command replace readonly with protected

[fedemp]

After reading the comment at #504 (comment) about properties that are marked as readonly when they should be writable, I realized that maybe you people meant protected when you used readonly here. Protected properties are writable but should only be modified by its own parent, not from the outside e.g. isEnabled that should be modified by refresh().

[Reinmar]

According to https://stackoverflow.com/questions/36843357/typescript-difference-between-private-and-protected-variables, these 2 props are not protected as they can be accessed from other classes. 

We (try to) follow these rules: https://ckeditor.com/docs/ckeditor5/latest/framework/guides/contributing/code-style.html#accessibility

In addition to that, we understand "readonly" as "it can be changed by the class itself, but not by the outside world". So @public/protected/private define visibility, while @readonly defines who can change it. 

I'm afraid that the semantic that we apply to these terms is different than what TS defined. In this case, when generating typings from our JSDoc comments, I'd suggest stripping information that's incompatible. Changing the meaning of these symbols across CKE5's entire code base is something that would need to be done thoroughly and would thus be a bigger task.

[fedemp]

No need to change ckeditor source then. But I would like to make it clear how TS should understand this.

Properties marked as readonly in ckeditor are expected to be modified only by the class and its subclasses, right?

At the same time, these properties can be read from the outside, right too?

If those two assumptions are fine, then the best typing would be like

    get foo():string;
    protected set foo(newvalue: string);

[Reinmar]

Properties marked as readonly in ckeditor are expected to be modified only by the class and its subclasses, right?

Yes, that's correct.

At the same time, these properties can be read from the outside, right too?

Yes.

If those two assumptions are fine, then the best typing would be like

As far as I understand, yes :) @arkflpc, could you confirm?

[arkflpc]

Yes, that's correct. The properties shouldn't be changed by Command clients but by derived classes. However, JSDoc accessibility attributes relate to the whole symbol, not setters and getters. That's why it's read-only.

[fedemp]

Excellent. I'll close this PR and i'll start doing a quick check on our typings for those readonly that should actually have a protected setter.