Add support for the Context and Watchdog features

[ma2ciek]

Suggested merge commit message (convention)

Feature: The <CKEditor> component contains the built-in watchdog feature. Closes #118.

Feature: Introduced the <CKEditorContext> component that supports the context feature.

Feature: Added the id property which is used to distinguish different documents. When this property changes, the component restarts the underlying editor instead of setting data on it, which allows e.g. for switching between collaboration documents and fixes a couple of issues (e.g. the onChange event no longer fires during changing the document). Closes #168. Closes #169.

Feature: The onError() callback will be called with two arguments. The first one will be an error object (as it was before the release 3+). A second argument is an object that contains two properties:

• {String} phase: 'initialization'|'runtime' - Informs when the error has occurred (during the editor/context initialization or after the initialization).

• {Boolean} willEditorRestart - When true, it means that the editor component will restart itself.

• {Boolean} willContextRestart- When true, it means that the context component will restart itself.

  The willEditorRestart property will not appear when the error has occurred in the context feature.
  The willContextRestart property will not appear when the error has occurred in the editor.

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• Add to the release summary: Both components (<CKEditor> and <CKEditorContext>) will internally use the Watchdog class that restarts the editor or context when an error occurs.

• Add to the release summary: The API of the entry point of the package has changed.

The previous way how the package was imported:

import CKEditor from '@ckeditor/ckeditor5-react';

will not work with the release (3+). Use:

import { CKEditor } from '@ckeditor/ckeditor5-react';

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BREAKING CHANGE: The entry point of the package has changed. The default import was removed since the package provides more than a single component now. Use import { CKEditor } from '@ckeditor/ckeditor5-react' instead of import CKEditor from '@ckeditor/ckeditor5-react';.

BREAKING CHANGE: The onInit property was renamed to onReady and can be called multiple times (after the initialization and after the component is ready when an error occurred).

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Additional information

[TODO]: Add a link to PR in ckeditor5.

[coveralls]

Pull Request Test Coverage Report for Build 309

• 61 of 68 (89.71%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-9.2%) to 90.789%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/ckeditorcontext.jsx	26	29	89.66%
src/ckeditor.jsx	35	39	89.74%

Totals
Change from base Build 231:	-9.2%
Covered Lines:	95
Relevant Lines:	102

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💛 - Coveralls

[ma2ciek]

I see there're some uncovered lines touching the error handling mechanisms. I'll cover them later today.

[pomek]

First-round. Mostly I am missing tests.

[pomek]

We need to decide about the AIP in the onReady/onError callbacks.

Also, minor refactoring would be nice.

Missing tests (and dropped coverage) could be fixed in a separate issue.

[pomek]

LGTM.