Make rich diff pluggable

Problem/Motivation

We have added image thumbnail support in #2825762: Show image thumbnail for image plugin

This is the first non-text diff and we might want to add more rich diffs components.
The current approach is hardcoded and thus contains a comment being EXPERIMENTAL. We need some kind pluggability to add more rich components.

Proposed resolution

Remaining tasks

Figure out how to make this satisfyingly pluggable.