+ Document and refactor the files using the new guidelines
* Now only non-technical names are used when building field chains and
domains (technical names can be used in debug mode).
* Better warnings are displayed when using the field and/or domain
selector widgets (also checks domain validity in both domain field
AND domain selector widget).
Also improve design (thanks to original work of @stefanorigano):
- Hovering 'remove' highlights the element that will be removed
- Hovering 'add node' btns shows where the new node will be created
- Restyle debug domain input field (make it similar to a text editor)
- Improve design and layout
- Resize inner fields