…e attributes (but not @Custom); ensure readingorder idrefs are correct (also for dummy text and image region)

- `TextractLayout`: init with additional list
  of top-level block dict stored in `child_regions`:
  - just ID→dict (for LAYOUT_ types during build)
  - or ID→block (for TextractTable, KEY, SELECTION etc.
     already instantiated)
- add `parent_layout` reference

- `TextractTable`: add `parent_layout` reference
- `TextractKey`: add `parent_layout` reference
- `TextractValue`: add `parent_layout` reference

- build `layouts` after all other blocks are built
- replace recursive `child_regions` / `parent_layout`
  after all `layouts` are built; remove recursive
  instances from top level to avoid duplication
  in ReadingOrder or PcGts instantiation

- ReadingOrder: extend recursive case (OrderedGroup)
  of `LAYOUT_FIGURE` with `LINE` children to `LAYOUT_*`
  with any `child_regions`

- instantiation of PcGts types for `layouts`
  and `tables`: refactor as (inline) function
  for recursion and
  - try to re-use code
  - add assertions around known types of recursion
    (to be revisited with better documentation from AWS
    or more data examples)