Basic support for clipboard (pasting HTML and plain-text)

[Reinmar]

TODO:

• https://github.com/ckeditor/ckeditor5-clipboard/issues/1 – The clipboard feature. DataTransfer facade + plain-text HTMLificiation + observers + end-feature (this time without buttons ;>). Perhaps also a command.
• Processing pipeline for input HTML. We'll use the editing pipeline, but we'll need to extend converter builders and standalone converters, because the editing pipeline currently works in one way only (model->view).
• composer.insertContent() (like CKE4's editor.insertHtml() + editor.insertText())
• Content filtering – tests, fixes and stability – moved to https://github.com/ckeditor/ckeditor5-clipboard/issues/3.

[Reinmar]

First 3 points were done. I wasn't able to work on filtering stability in this release so I moved it to a separate ticket (https://github.com/ckeditor/ckeditor5-clipboard/issues/3) and we'll work on it in the next iteration.