Split MessageEditor in edit-specifics & reusable part for main composer

src/editor/model.js

@@ -24,9 +24,17 @@ export default class EditorModel {
         this._activePartIdx = null;
         this._autoComplete = null;
         this._autoCompletePartIdx = null;
+        this.setUpdateCallback(updateCallback);
+    }
+
+    setUpdateCallback(updateCallback) {
         this._updateCallback = updateCallback;
     }
 
+    get partCreator() {
+        return this._partCreator;
+    }
+
     clone() {
         return new EditorModel(this._parts, this._partCreator, this._updateCallback);
     }