refactor: change meta class structure to include types.

Introduce requiredness of translations via implemented interface.

src/core/things/Diff.ts

@@ -15,6 +15,7 @@
 import * as jsonpatch from 'json-patch';
 import {SCThingsWithoutDiff} from '../Classes';
 import {SCThing, SCThingMeta, SCThingType} from '../Thing';
+import {SCMetaTranslations} from '../types/i18n';
 import {SCISO8601Date} from '../types/Time';
 
 /**
@@ -60,15 +61,40 @@ export interface SCDiff extends SCDiffWithoutReferences {
 }
 
 /**
- * Diff meta data
+ * Meta information about a diff
  */
-export class SCDiffMeta extends SCThingMeta {
-  static fieldTranslations = {
-    ...SCThingMeta.fieldTranslations,
+export class SCDiffMeta extends SCThingMeta implements SCMetaTranslations<SCDiff> {
+  /**
+   * Set type definiton for singleton instance
+   */
+  protected static _instance: SCThingMeta;
+
+  /**
+   * Translations of fields
+   */
+  fieldTranslations = {
     de: {
+      ... SCThingMeta.getInstance().fieldTranslations.de,
       action: 'Aktion',
       changes: 'Änderungen',
       dateCreated: 'Erstellungsdatum',
     },
+    en: {
+      ... SCThingMeta.getInstance().fieldTranslations.en,
+    },
+  };
+
+  /**
+   * Translations of values of fields
+   */
+  fieldValueTranslations  = {
+    de: {
+      ... SCThingMeta.getInstance().fieldValueTranslations.de,
+      type: 'Unterschied',
+    },
+    en: {
+      ... SCThingMeta.getInstance().fieldValueTranslations.en,
+      type: SCThingType.Diff,
+    },
   };
 }