Create a reusable dialog class that can display a JFXPanel

[cwisniew]

Create a reusable dialog class that can display a JFXPanel.
The standard JavaFX dialogs can not be used as they live in a separate hierarchy to Swing windows so can not be modal with the main MapTool frame, nor will they stay above the MapTool frame if it is clicked on.

[cwisniew]

This was included as part of PR #2992

Added several classes in https://github.com/RPTools/maptool/tree/develop/src/main/java/net/rptools/maptool/client/ui/javfx
These are

• FXMLLoaderUtil - Utility class for loading FXML files.
• SwingJavaFXDialog - A Class that shows JavaFX content in a Swing dialog from FXML, takes care of most of the threading issues for you.
• SimpleSwingJavaFXDialog - An implementation of SwingJavaFXDialog that provides a call back to obtain controller and registering close events. In most cases this is the class you will want to implement.
• SwingJavaFXDialogController - interface that controllers for SimpleSwingJavaFXDialog FXML content must implement.
• SwingJavaFXDialogEventHandler - Event handler interface for listening for close events.

A couple of examples on how to use the classes above can be found in
https://github.com/RPTools/maptool/tree/develop/src/main/java/net/rptools/maptool/client/ui/players