Support sub-patterns for block

Problem/Motivation

Currently we support assigning the block content to a pattern. There are cases however where it would be valuable to be able to configure the sub-elements of a block. An example is the system_branding_block. The render array includes up to three sub-elements, 'site_logo', 'site_name', and 'site_slogan'. Allowing configuration of these sub-elements would for example allow assigning a text pattern to 'site_name', which might enable selecting a font, font size, and so on.

Proposed resolution

In the form builder, build the block and configure a pattern selector for each valid child of the render array.

In the block display alterer, selectively wrap child elements in patterns.

Remaining tasks

Will require a fix for an issue in UI Patterns, Cannot include two pattern select elements in same form. Pending that we could implement a workaround.

User interface changes

API changes

Data model changes