Added shorthand focus() method in generic Editor class

[psmyrek]

Suggested merge commit message (convention)

Feature (core): Introduced the focus() method in the base Editor class. Closes #714.

---

Additional information

The editor.focus() method is just a shorthand for editor.editing.view.focus().

[mlewand]

the CI fails with:

WARN: 'Detected `console.warn()`:', 'There is no selection in any editable to focus.'
Chrome 86.0.4240.111 (Linux x86_64) Editor focus() should call view's focus() method FAILED
	Error: Detected `console.warn()`:
	    at console.<computed> [as warn] (webpack:///./build/.automated-tests/entry-point.js?:40:11)
	    at View.focus (webpack:///./packages/ckeditor5-engine/src/view/view.js?:438:33)
	    at Function.invoke (node_modules/sinon/pkg/sinon.js:1922:47)
	    at View.focus (node_modules/sinon/pkg/sinon.js:2178:26)
	    at TestEditor.focus (webpack:///./packages/ckeditor5-core/src/editor/editor.js?:19:4566)
	    at Context.eval (webpack:///./packages/ckeditor5-core/tests/editor/editor.js?:577:11)

https://travis-ci.org/github/ckeditor/ckeditor5/builds/738967351

[psmyrek]

CI build has been fixed by explicitly setting editor.editing.view.document.isFocused = true in the unit test, because the only thing this particular test is checking is that whether it calls the focus() method on the view - regardless of its real implementation details, which is already tested.

[mlewand]

LGTM, just one last thing that we should do in this PR is to replace existing editor.editing.view.focus() calls with the new, nicer form in our codebase.

[mlewand]

Please check the context of remaining changes.

[mlewand]

LGTM, minor adjustment in one guide.