Add event subscription methods for Editor and associated classes

* [ ] Add event subscription methods
  * [ ] :wrench: Editor
  * [x] Cursor
  * [x] Selection
  * [x] Decoration
  * [ ] DisplayBuffer
  * [x] TokenizedBuffer
* [ ] Deprecate `::on`
  * [ ] Editor
  * [x] Cursor
  * [x] Selection
  * [x] Decoration
  * [ ] DisplayBuffer
  * [x] TokenizedBuffer
* [ ] Document new methods

### Notes

* We should drop events on editor that can be replaced by methods on `Cursor`, `Selection`, `Decoration`
* Don't forget to dispose emitter instances on destruction of the owner
Was able to remove all this junk as the EditorComponent now directly subscribes to the decorations when they are added. Nice to remove this stuff...