Improve layout configuration for easier decoration

In the the current layout configuration

  rows:
    - cssClass: editor
      groups:
        - name: content
          apps:
            - pages-app
            - dam
            - stories
            - tours
            - contacts
        - name: marketing
          apps:
            - analytics
            - campaigns
            - marketing-tags
            - abn-testing 

The groups (the apps inside each group) are defined inside a row. While this approach allows easily seeing the definition and configuring all the layout with the resource file app, it is not that easy to decorate it, for instance, if we want to move a group to a different row, we have to define its apps again.

Moreover, when the apps list is decorated, instead of appending the new value into the list, the new value is replacing the 1st element already defined in the list. (this is also valid for hidden apps).

We want to change the definition in order to:

• Have the groups defined separately of the rows, so the decoration is easier to apply
• Add names in most groups, so it is possible and easier to decorate just one entry. This also fixes the issue of replacing the 1st item of the list when using decoration.
• Enable decorating the hidden apps without replacing the existing apps.

Acceptance criteria

• It is possible to decorate a group adding an app keeping the existing apps

layout:
  groups:
    - name: content
      apps:
        - name: new-app

In this case, new-app will be added into the list without replacing pages-app.

• It is possible to decorate the hidden apps keeping the existing apps

layout:
  hiddenApps:
    configuration: configuration 

• It is possible to hide a row

layout:
  rows:
    - name: one
      displayed: false