Initial implementation of the clipboard feature

[Reinmar]

Fixes ckeditor/ckeditor5#2647.

Status of things:

1. It works fine in Chrome and Firefox. You can paste content within the editor or from outside. In Safari, only plain text is available so far, but https://bugs.webkit.org/show_bug.cgi?id=19893 is finally closed so this should start to work... one day.
2. Handling spaces is really tricky. See Handle whitespaces ckeditor5#2651.
3. Pasting some disallowed content will lead to crashes, inf loops, etc. See Content filtering stability ckeditor5#2649.