Update return type of createMediator method

Author: pcastornii

mvvm4fx-core/src/main/java/com/techsenger/mvvm4fx/core/AbstractComponent.java

@@ -290,5 +290,5 @@ protected void setHistoryProvider(HistoryProvider historyProvider) {
         this.historyProvider = historyProvider;
     }
 
-    protected abstract AbstractComponent.Mediator createMediator();
+    protected abstract Mediator createMediator();
 }

mvvm4fx-core/src/main/java/com/techsenger/mvvm4fx/core/AbstractParentComponent.java

@@ -108,7 +108,7 @@ public ObservableList<ChildComponent<?>> getChildren() {
     }
 
     @Override
-    protected abstract AbstractParentComponent.Mediator createMediator();
+    protected abstract Mediator createMediator();
 
     protected void addChild(ChildComponent<?> child) {
         this.modifiableChildren.add(child);