VirtualContainerBase: createVirtualFlow must be protected

Summary

API visibility change is done to facilitate custom VirtualFlow creation in JavaFX skin classes.

Problem

It is impossible to provide a custom VirtualFlow in JavaFX skin classes in JDK 9. On pulling VirtualContainerBase class into public scope in JDK 9, createVirtualFlow() method changed visibility from protected to package-private, thus fighting Its own api intent.

Solution

Following methods in VirtualContainerBase class are made protected to allow custom VirtualFlow creation in JavaFX skin classes - 1. createVirtualFlow() 2. getVirtualFlow()

Specification

    /**
     * Create the virtualized container that handles the layout and scrolling of
     * all the cells. This enables skin subclasses to provide 
     * a custom {@link VirtualFlow} implementation.
     * If not overridden, this method intantiates a default VirtualFlow instance.
     * @return newly created VirtualFlow instance
     * @since 10
     */
    protected VirtualFlow<I> createVirtualFlow() {...}


    /**
     * Get the virtualized container.
     * Subclasses can invoke this method to get the VirtualFlow instance.
     * @return the virtualized container
     * @since 10
     */
    protected final VirtualFlow<I> getVirtualFlow() {...}