Simplify using images in component templates

[mhaack]

Currently using images in component templates is cumbersome, like:

"plugins": {
        "da": {
          "name": "carousel",
          "unsafeHTML": "<div><div><div><picture><source type=\"image/webp\" srcset=\"\"><source type=\"image/jpeg\" srcset=\"\"><img loading=\"lazy\"></picture></div><div></div></div>"
        }
      }

We should support a simple placeholder <picture> which is replaced by UE with the full picture tag. The configuration would look like:

"plugins": {
        "da": {
          "name": "carousel",
          "unsafeHTML": "<div><div><div><picture></div><div></div></div>"
        }
      }

The backend can detect the <picture> tag without any children or extra attributes and expand it with a default template. This way we can stay backward compatible easily.