Extending multiple blueprint at once#378

As you know, it is not possible to make more than one extending at the same level at the moment.

When you write a modular system, you separate different parts of a field to avoid code duplication and call them for different uses in different places.

fields:
  layout:
    type: layout
    label: Layout
    extends:
      - layout/layouts
      - layout/fieldsets
      - layout/settings