[META] Fundamental Type: Separator

Problem/Motivation

Sometimes a special visual separator makes sense.
This could be covered by a special stle, but then a visual separator is not independent of other components.

Proposed resolution

Create a new paragraph type that is only a separator.
We could offer multiple styles for it (say, a bar, some nice vector art, ...)

The vector art could ask for some similar picker like the icon picker.
#2830016: Add a thumbnail/icon field to Paragraphs type

The Anchor plugin could be simply only enabled in that separator. To only offer an invisible anchor, we could simply also an invisible separator style.
#2851403: New Plugin: Anchor

We could also use this separator item to create an outline with jumpmarks like it's typical for some long single-pagers.
Some sites alternatively put the title of the current chapter into the page header (block).

Let's keep this issue as meta and discuss all the ideas and create child issues for specific actions.