feat!: flatten pageType registration configuration structure

[schloram]

With this change we are getting rid of the configure config sub-level. I didn't really understand why we separated the config like this in the first place.

old:

dokType: 87
configuration:
  label: 'My Custom pageType'
  iconSet:
    defaultIcon:
      source: 'EXT:your_ext/Resources/Public/Icons/apps-pagetree-mycustompagetype.svg'
    hideInMenuIcon:
      identifier: 'apps-pagetree-page-frontend-user-hideinmenu'
    rootPageIcon:
      source: 'EXT:your_ext/Resources/Public/Icons/apps-pagetree-mycustompagetype-root.svg'
  isDraggableInNewPageDragArea: true

new:

dokType: 87
label: 'My Custom pageType'
iconSet:
  defaultIcon:
    source: 'EXT:your_ext/Resources/Public/Icons/apps-pagetree-mycustompagetype.svg'
  hideInMenuIcon:
    identifier: 'apps-pagetree-page-frontend-user-hideinmenu'
  rootPageIcon:
    source: 'EXT:your_ext/Resources/Public/Icons/apps-pagetree-mycustompagetype-root.svg'
isDraggableInNewPageDragArea: true

In perspective of making the default icon optional (#26) the minimal pageType setup would be as simple as:

dokType: 87
label: 'My Custom pageType'

instead of:

dokType: 87
configuration:
  label: 'My Custom pageType'