Make the viewModel field final

Author: pcastornii

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

@@ -27,7 +27,7 @@ public abstract class AbstractComponentView<T extends AbstractComponentViewModel
 
     private static final Logger logger = LoggerFactory.getLogger(AbstractComponentView.class);
 
-    private T viewModel;
+    private final T viewModel;
 
     private ComponentComposer<?> composer;