Emitter#stopListening() and Emitter#off() improvements

[scofalik]

Suggested merge commit message (convention)

Fix: utils.Emitter#off() called without arguments will stop executing all callbacks added by utils.Emitter#on(). Additionally, utils.Emitter#stopListening() called without arguments will also call utils.Emitter#off() thus stop all callbacks added by utils.Emitter#on(). Closes ckeditor/ckeditor5#4947.

[Reinmar]

I pushed a couple of improvements, but unfortunately, I find this module too messy. We're just asking for troubles. You can even see that the currently implemented solution is a hack by how it stands out from the rest of the code: https://github.com/ckeditor/ckeditor5-utils/pull/145/files#diff-1b61a350b30716326bca3d6e478665d8R219

For me, this.on() must use this.listenTo( this, ... ). This may need some refactoring because today the on() method is the base one, while listenTo() uses it. For me, it should be the opposite.

I know that we could live with the proposed solution for months, but for me this is too important piece of the code to have any doubts about how it works. We can see that it already leaked and that all of us were surprised what happened (even though I found this in the past). So, since the entire editor bases on this mechanism I'm for keep it as clean as possible.

[Reinmar]

R-, but I'm open for discussion ofc :P

[scofalik]

I, too, hate this whole mixin. But this PR blocks this: ckeditor/ckeditor5-engine#904 . Maybe we could refactor it in separate issue?

[Reinmar]

But this PR blocks this: ckeditor/ckeditor5-engine#904 . Maybe we could refactor it in separate issue?

This issue isn't critical so we're not in hurry. Having it open and having this one open will force us to get back to this topic sooner.

[scofalik]

I've made some improvements on branch 144-b but they are not ready yet. I will be tinkering with this in "meantimes".

[Reinmar]

#190 replaces this PR so I'm closing this one.