IMPRO,FEAT: added modifyChildren

enables passing a modifier to all children of a flex container.
Similar to distributeSpacingEvengly.
might be refactored into component in general.

src/component/FlexContainerComponent.js

@@ -130,6 +130,33 @@ class FlexContainerComponent extends Component {
         return this;
     }
 
+    /**
+     * Adds, sets, updates or overwrites the Modification of/for the children of this component.
+     * Just calls .modifier(modifier) on each child. 
+     * 
+     * @param {Modifier} modifier 
+     * @param {boolean|ExtStorage|ExtStoreType|OverwriteBehaviour} [extStore=null] 
+     * @returns {FlexContainerComponent} this component object
+    */
+    modifyChildren(modifier, underTheName = "", extStore = false) {
+        if (underTheName === "") {
+            underTheName = `.${this._compName}-style-children`;
+        }
+        if (!extStore) {
+            for (const child of this._children) {
+                child.modifier(modifier)
+            }
+        } else {
+            this.addStyleClass(underTheName);
+            this._styles.push(
+                new SStoreDefinition(
+                    underTheName,
+                    modifier.ensureModifier()
+                )
+            );
+        }
+        return this;
+    }
 
     /**
      * @override