Provide helpers for a useful and good-looking backend preview

[jonaseberle]

This could be done via

• a layout
• partials
• ViewHelpers
• a JavaScript-API?

Acceptance criteria (draft)

• [ ] minimal configuration enables a usable preview
• [ ] integrators can select which fields are represented in the preview
• [ ] integrators can override field representations for a content block (CType, fieldname)
• [ ] integrators can override field representations for all content blocks (field type)

Acceptance tests

• [ ] An partial for every field type exists
• [ ] It is possible to add an own partial for each field type to an existing sitepackage and it is used
• [ ] It is possible to add an partial to a contentBlock which is used

Preconditions / boundary conditions

• [ ] the content block registration api shifts partial files for every field type
• [ ] the partial could be overridden from every sitepackage
• [ ] every partial could be overridden from every contentBlock