Preview of view sources

Problem/Motivation

View integration don't show preview of view sources, only placeholders.

Proposed resolution

For each view source, execute the view and extract the rendered plugin.

It may be easier to start from 📌 Dynamic theme registry alteration Active and it may make easier to do this later: ✨ Expose view sources forms in SourceInterface::settingsForm() Active (because the logic to load and run the specific plugin could be shared).