Display type-specific icons dynamically

[tusmester]

Currently the icons shown in the tree and grid of admin ui are hardcoded.

As an editor I want to define the icon of a content type in the CTD and see them automatically in admin ui.

• there is already an Icon value in the CTD header, we can use that, hopefully no backend changes are needed.
• we define icons in the CTD:
  • absolute image url (e.g. https://othersite/icons/myicon.png) (can point to anywhere, e.g. the repository itself or a 3rd party cdn)
  • icon name (e.g. article) that is resolved on the client. From where?
    • icon folder in the repository?
    • from a font on the client?
    • ?