Block layout import

Problem/Motivation

Currently when creating a page layout with Display Builder, default layout is imported from modules/display_builder_page_layout/fixtures/default_page_layout.yml, if it is the first page layout, let's try to import from current block layout configuration.

Because currently, a user could ask:
- why is there the XXX block while I have removed it?
- where are all the blocks I have configured on block layout?

Also currently, the fixture references some blocks generated by derivatives from config, menu block, main, account, footer. What if there is no footer menu on the website? (very low probability that there is no main and account menu even if theorically possible)

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes