Allow to specify a Layout template type

[laryn]

Description of the need

This is one step towards #6039

In my spare time, I've been (slowly) working on a Layout Paragraphs solution that utilizes Backdrop's Layout system rather than mimicking the Drupal way of doing it. The issue above also links to some other use cases for this.

I'd like to be able to extend Layouts without having to completely duplicate all the functionality by being able to specify a "Layout Template Type" that can be treated separately. This allows layouts and layout templates that are intended for other uses not to get pulled in to visually pollute the core/main layouts and layout template listings.

Proposed solution

Core would continue to provide (and find) standard layout templates as well as flexible layout templates. Functionality would be extended by provided the expected layout template types to various functions and functionality as defaults so core continues to function as it does, but allows contrib to use many of these functions with different inputs.

Alternatives that have been considered

Duplicating and slightly changing every single layout-related function/class/etc to provide layout-related functionality in contrib. Duplication may still need to happen to some degree even with the above, but much less.

Draft of feature description for Press Release (1 paragraph at most)

Backdrop now includes the ability for developers to create new Layout template types in addition to the core 'layout' and 'flexible' template types.