Basic features: editing hints, pre-set languages, customizable blocks

[bikubi]

A set of features+improvements that make the plugin usable for us.
I can split it up into 3 or 4 PRs if you'd prefer that.

1. display lang attribute value (the code) in the editor
   related to [Improvement] Add visual queue to blocks that have a lang attribute set on the Document overview #1, but in the WYSIWYG area
2. a filter to pre-set language codes
   Related to [Improvement] Locales dropdown instead of free input field for language #5, but not all languages. Should not interfere.
3. a filter to add/customize supported blocks
   I thought I saw a TODO somewhere for that... might need some work to extend beyond core/paragraph.

Thank you for the plugin, I like the simplicity and code quality!

[webdados]

Hi @bikubi, and thanks for contributing.

Could you please split this into a PR for each change?