[2.0.0-beta2] Layout Builder: Restore icon_path & icon_map; add thumbnailPath

Problem/Motivation

We need to restore this feature used by Layout Builder:

  $layout = new LayoutDefinition($definition);
    if (isset($component['icon_map'])) {
      $layout->setIconMap($component['icon_map']);
    }
    if (isset($component['thumbnailPath'])) {
      $layout->setIconPath($component['icon_path']);
    }
    if (isset($component['icon_path'])) {
      $layout->setIconPath($component['icon_path']);
    }

Example of icon_map in a component definiton:

  icon_map:
    - [top]
    - [first, second]
    - [bottom]

And it is also the opportunity of leveraging thumbnailPath in layout builder:

1. if you put a file named thumbnail.png in your component folder, SDC is setting thumbnailPath property in definition
2. if we have thumbnailPath and not icon_path in the definition, icon_path = thumbnailPath

Proposal:

  $layout = new LayoutDefinition($definition);
    if (isset($component['icon_map'])) {
      $layout->setIconMap($component['icon_map']);
    }
    if (isset($component['thumbnailPath'])) {
      $layout->setIconPath($component['icon_path']);
    }
    if (isset($component['icon_path'])) {
      $layout->setIconPath($component['icon_path']);
    }

Related issue, but for component selector instead of layout builder #3469795: [2.2.0] Component selector: use SDC's thumbnailPath?