Reorganise dependencies and devDependencies

[Reinmar]

Related to ckeditor/ckeditor5-design#112

It should be possible to use gulp build when the ckeditor5 package is installed as your project's dependency. This means that we need to move some dependencies from dev to normal ones.

[Reinmar]

At the same time ckeditor5-* (other than ckeditor5-core which is a hard dependency) dependencies which we have defined in the dependencies section should be moved to devDependencies, because we only want them installed while we're developing the thing. (But there are no other deps right now, so no changes are needed :D).

[Reinmar]

OK, I pushed branch t/74 but I cannot test that gulp build works, because existing tasks try to load non-existing modules. We would need to start including modules inside tasks, which isn't that bad idea taken that I've read that requiring so many modules hits the IO hard and slows down gulp tasks heavily.

[Reinmar]

We focused now on making it possible to fork ckeditor5 or install ckeditor5-dev-* packages directly in your project. This means that it's not clear how dependencies in ckeditor5 must be split because it's only a dev environment for CKEditor 5, so it should always be used in a dev mode.