Decouple engine from UI

[phenaproxima]

I have a use case wherein I would like to be able to slap drupal-entity tags into my CKEditor, but without using Entity Embed's UI at all, and without its button(s) appearing in the editor.

This PR decouples the "engine" part of Entity Embed -- i.e., the widget definition -- from the UI and command parts. They are split into two separate JS files -- engine.js and plugin.js -- and two separate plugins on the Drupal side (drupalentity and drupalentity_button).

[mirodietiker]

That sounds really interesting and i think if entity_embed wants to offer nice helpers for all use cases we need this kind of separation.

[slashrsm]

Great work! I agree with @mirodietiker. This change makes a lot of sense. I gave it a quick sanity check only as I don't have time to do a proper review ATM. I hope me or somebody else comes back to properly review and test asap.

[webflo]

👍

[balsama]

This is great - and likely a common use case. +1

[davereid]

Please, I'd love to review this as well.

[davereid]

Seems odd that this would still extend EmbedCKEditorPluginBase?

[slashrsm]

This is strange indeeed.

[slashrsm]

I tested this PR and it seems to breaks CKEditor.

Could we convert "engine" into a JS library? Would this allow us to use it in button plugin without extending "engine" class.

This PR is definitely a step in the right direction, but it needs some more love.