Introduced the editor quick actions toolbar

[oleq]

Feature: Introduced editor quick actions toolbar (see #38) to log editor data, toggle editor read-only, and destroy editor. Closes #121. Closes #104.

---

Notes

Under the hood, 

• I refactored the Button component to be more versatile because the number of buttons increased and some need isOn/isEnabled states. 
• Also, I changed the way icons are handled in the project (loaded from SVG instead of hardcoded base64), unified their size, and made them compatible with mentioned buttons.
• Toolbar separators are spans with a class now for reusability because they appear in two different places.

[psmyrek]

LGTM 🎉

I briefly scanned the CKEditor 5 Inspector code (as this was the first time I saw it), then I ran this PR and it works as expected.

The only one thing to consider is updating the screenshot of the Inspector in the docs after releasing new version: https://ckeditor.com/docs/ckeditor5/latest/framework/guides/development-tools.html#ckeditor-5-inspector. I don't know if it appears elsewhere in the docs.

[przemyslaw-zan]

Should the editor toolbar stay after destroying it? This happens for me for all three editors in the sample.

[oleq]

Should the editor toolbar stay after destroying it? This happens for me for all three editors in the sample.

This is how decoupled document editor works. The integrator takes care of injecting the toolbar and removing it. It's just a sample so I didn't bother.

[oleq]

The only one thing to consider is updating the screenshot of the Inspector in the docs after releasing new version: https://ckeditor.com/docs/ckeditor5/latest/framework/guides/development-tools.html#ckeditor-5-inspector. I don't know if it appears elsewhere in the docs.

README.md of the inspector on Github/Npm, that's for sure.

[pomek]

I extracted updating documentation to a new issue – #133.

[pomek]

LGTM.

[przemyslaw-zan]

LGTM