Virtualized controls: add api to support semantic prefHeight computation

currently the skins hard-code it to 400:

    @Override protected double computePrefHeight(double width, double topInset, double rightInset, double bottomInset, double leftInset) {
        return 400;
    }

Instead, the default computation should be based on # of rows to show. A first step could be to delegate to VirtualFlow. Which takes the cellCount, unfortunately max'ed to a hard-coded 10.