☂ Fix snapping for layout container

With JDK-8296330 ( https://github.com/openjdk/jfx/pull/936 ), we will have a stable test base to test the snapping of all layout container.
The next step is to fix all broken layout container.
Another step to improve the performance is to use the snappedXXXInsets methods on all working layout containers.

Working layout container:
- AnchorPane

Partially working layout container which also can still be optimized by the snappedXXXInsets methods:
- HBox
- VBox
- GridPane

Non-Working layout container, where the insets needs to be correctly snapped by the snappedXXXInsets methods:
- Pane
- StackPane
- BorderPane
- TilePane
- FlowPane