Features should set their keystrokes through the keystroke handler

[Reinmar]

This is an extension of https://github.com/ckeditor/ckeditor5-core/issues/20.

Features like the enter key, delete and list (tab to indent) listen on the view. This makes it harder to change keystroke which triggers them. In CKEditor 4 you were able to define that the enter command is triggered by any key. Sometimes this may be useful. Perhaps not this case, but I can see more people redefining indent/outdent keystrokes.

Edit: Please read #340 for more details.

Another thing to consider is that perhaps actions which will have their associated beforeinput type should not be set through the keystroke handler. This, in the future, would be a serious problem, because we'd need to stop listening to the keystroke and start listening to the beforeinput event (you can do both at the same time). After all, if someone wants to change some key behaviour, then the keystroke handler will always be available (keydown should be fired first).

tl;dr:

1. Keystrokes which have their associated beforeinput type should be handled directly through the view#keydown event.
2. All other keystrokes should be handled through the keystroke handler.

[Reinmar]

We kept defining the "typing" keystrokes (typing, delete, enter) via the editing view and I think now that it was a sensible choice. These keystrokes are not to be changed just like that. They are part of the "essentials" feature which recreates a textarea-like behaviour. We can say that this is the stable ground on which editing features are built and these editing features can use (and do that) the keystroke manager.