[1.0.0-beta2] Migration from Layout Builder

Problem/Motivation

When a website is already using Layout Builder in config (entity view display) or in content (overrides), it would be nice to keep as much as display config possible while switching to Display Builder.

Proposed resolution

Layout Builder has a 2 levels only data structure, so it may not be hard to map to our tree-like data structure.

First level: Layout Builder sections:

• they can host only Layout Plugins
• if the Layout is a SDC component (thanks to ui_patterns_layouts) >> ✅ We have a Component source
• if the Layout is not an SDC component >> ⚠️ we will need this source: ✨ [1.0.0-beta2] Layout plugins support Active
• Styles can be put at the section/layout level >> ✅ we can move them to our _third_party_settings/ui_styles config
• Styles can also be put in region level >> ⚠️ what do we do? Is it a missing feature from Display Builder?

Second level's Layout Builder "components" (not related to SDC components):

• they can host Block plugins >> ✅ We have a Block source
• which are sometimes wrapping a field formatter plugin >> ✅ We have a Field source
• which are sometimes wrapping a SDC component (thanks to ui_patterns_blocks) >> ✅ We have a Component source
• Styles can be put at the block level >> ✅ we can move them to our _third_party_settings/ui_styles config