Allow top-down frontend customisation

Right now you can customise the frontend bottom-up. So starting from interfaces/displays, and if the modification is larger, a new layout, and if even larger, a module, and if even larger a whole new frontend app.

This is a suggestion to allow top-down customization too, allowing the developer to use existing layouts/modules as part of a custom app. This can be done by allowing the developer to write custom private or public views.

A good first step to that is to register the built-in modules, layouts and [the private view components]( This will allow building custom views with whatever branding aspects are needed as part of a new custom module, then simply plugging in existing components/layouts/modules.

This would also close #4204.