[Umbrella] Engine refactoring

[pjasiun]

Topics to be considered:

• Remove deltas- will be fixed later, not as a part of this umbrella
  • Tickets: https://github.com/ckeditor/ckeditor5-engine/issues/500, https://github.com/ckeditor/ckeditor5-engine/issues/1162
• Simplify the change event and batch API
  • Tickets: https://github.com/ckeditor/ckeditor5-engine/issues/679
• Merge controllers
  • Tickets: https://github.com/ckeditor/ckeditor5-engine/issues/678
• One API to rule them all
  • Tickets: https://github.com/ckeditor/ckeditor5-engine/issues/858, https://github.com/ckeditor/ckeditor5-engine/issues/738, https://github.com/ckeditor/ckeditor5-engine/issues/1186, https://github.com/ckeditor/ckeditor5-engine/issues/1209, https://github.com/ckeditor/ckeditor5-engine/issues/1210
• Get rid of enqueueChanges() / Post-fixer API
  • Tickets: https://github.com/ckeditor/ckeditor5-engine/issues/654, https://github.com/ckeditor/ckeditor5-engine/issues/1101, https://github.com/ckeditor/ckeditor5-engine/issues/1207
• Markers are all you need
  • Tickets: https://github.com/ckeditor/ckeditor5-engine/issues/1086
• Schema improvements
  • Tickets: https://github.com/ckeditor/ckeditor5-engine/issues/532
• Conversion improvements
  • Tickets: Epic (https://github.com/ckeditor/ckeditor5-engine/issues/829, https://github.com/ckeditor/ckeditor5-engine/issues/736, https://github.com/ckeditor/ckeditor5-engine/issues/778, https://github.com/ckeditor/ckeditor5-engine/issues/779, https://github.com/ckeditor/ckeditor5-engine/issues/945, https://github.com/ckeditor/ckeditor5-engine/issues/1214)
• Reduce memory pressure by stopping cloning all the time
  • Tickets: https://github.com/ckeditor/ckeditor5-engine/issues/897

[scofalik]

We were talking about making conversion on changesDone event. I can't find exact issue or even if we finally all agreed this is a good idea. Here is the issue I found: https://github.com/ckeditor/ckeditor5-engine/issues/829

[Reinmar]

I also think that https://github.com/ckeditor/ckeditor5-engine/issues/897 (stop cloning everything, enforced immutability) is a part of this.

[Reinmar]

I'll add https://github.com/ckeditor/ckeditor5-engine/issues/1038 cause it grew to a significant problem and it turns out that it's not a quick and invisible change too (@scofalik can tell you more).

[pjasiun]

Finally!