Add Gutenberg related styles and features

[Zodiac1978]

Popper is not well prepared for Gutenberg at the moment.

As a first step we could add some more styles to style some of the block editor modules, like the separator block. Image and Gallery block, Drop Cap, etc.

Furthermore we could maybe enable theme support for align-wide and align-full.

Anders Noren has prepared all of his themes with Gutenberg styles, so this could be a starting point:
https://themes.svn.wordpress.org/hemingway/1.67/style.css

[Zodiac1978]

There is a well written articel about align-wide and align-full which could be a good starting point:
https://www.billerickson.net/full-and-wide-alignment-in-gutenberg/

[Zodiac1978]

Maybe we could also add some Block Styles or Block Patterns.