Support for keyup event on document

[pomek]

In order to detect whether the user keeps the Backspace key, we need to add support for the keyup event called on Document.

It's required in order to implement a test case II from here – https://github.com/ckeditor/ckeditor5-typing/issues/61#issuecomment-314705304.

[Reinmar]

We agreed on a different solution – that the keydown could have a "sequence" property with the number identifying thre 1st, 2nd, etc. press (without release).

[Reinmar]

OTOH, exposing keyup may be more flexible. But you'll need much more code in ckeditor/ckeditor5-typing#61 to handle that.

OTTH... we'd actually need to count which exactly key does this sequence apply to. So perhaps that generic sequence in keydown isn't a good idea. E.g. ctrl+backspace is already two keydowns but you'd expect that sequence is 1 ;/

Let's better add the keyup then and have the code inside the typing feature.

[Reinmar]

There's also the third option – to have keyup fired by the KeyObserver and the sequence handled by the DeleteObserver. Let's do this.