Add event subscription methods for Editor and associated classes

* [ ] Add event subscription methods
  * [ ] :wrench: Editor
  * [x] Cursor
  * [x] Selection
  * [x] Decoration
  * [x] DisplayBuffer
  * [x] TokenizedBuffer
* [ ] Deprecate `::on`
  * [ ] Editor
  * [x] Cursor
  * [x] Selection
  * [x] Decoration
  * [x] 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
A user could conceivably destroy the decoration without destroying the marker, then the marker would have a longer lifespan. So this will unsub itself in that case.