Theming changes for 5.0.x

Cleanup some things related to theming:

• 📌 Change layout templates to use content.region (with BC) Active : stop copying over from 'content' to the root. This will affect templates to use {{content.region_name}} (like layout discovery does.)
• 🐛 Template suggestions does not work Active : review template suggestions in general
• #2895316: Parent theme template inheritance bug → : obsolete for 5.0.x and I'm not going to commit that patch to 8.x-3.x as I'm to afraid for that, but ds_theme_registry_alter is weird to say at the least.
• #2888366: fields customization not working → : figure out what's wrong there, and possibly generalize field theming through DS
• 🐛 Make field template suggestions and preprocess work Active : read and understand what I wrote down there a long time ago. :)