Many javafx scenegraph classes have implicit no-arg constructors

The following scenegraph classes in the javafx.graphics and javafx.controls modules have implicit no-arg constructors, which are added by javac, and show up in the API docs without any documentation associated with it:

javafx.application.Preloader
javafx.concurrent.ScheduledService
javafx.css.PseudoClass
javafx.css.Selector
javafx.css.StyleConverter
javafx.css.StyleConverter.StringStore
javafx.css.converter.ShapeConverter
javafx.scene.control.TableFocusModel
javafx.scene.control.TableSelectionModel
javafx.scene.input.ClipboardContent
javafx.scene.shape.PathElement
javafx.scene.transform.Transform
javafx.stage.DirectoryChooser
javafx.stage.FileChooser
javafx.stage.Popup

The fix is to add an explicit constructor with minimal documentation.

This is part of a larger effort to eliminate such classes. See JDK-8250558.