Simplify model hierarchy of field description and other localizedStructuredContent fields

[morpheus-87]

Now:

"description": {
      "localizedStructuredContent": {
        "en": {
          "type": "doc",
          "content": [
            {
              "type": "paragraph",
              "content": [
                {
                  "type": "text",
                  "text": "Englische Beschreibung"
                }
              ]
            }
          ]
        }
      }
    }

Simplified without localizedStructuredContent key:

"description": {
        "en": {
          "type": "doc",
          "content": [
            {
              "type": "paragraph",
              "content": [
                {
                  "type": "text",
                  "text": "Englische Beschreibung"
                }
              ]
            }
          ]
      }
    },

LocalizedStructuredContentImpl should extend HashMap.

[morpheus-87]

Done with #64.