Update clear method to clear in navbar

Author: kevzlou7979

gwt-material/src/main/java/gwt/material/design/client/ui/MaterialNavBar.java

@@ -97,6 +97,11 @@ public void add(Widget child) {
         div.add(child);
     }
 
+    @Override
+    public void clear() {
+        div.clear();
+    }
+
     @Override
     public void setType(NavBarType type) {
         typeMixin.setType(type);