Adding new features to FxApplication

Several new features included:

• Adding accelerator test mode: channels get a suffix appended automatically for testing purposes.
• New methods setup and beforeStart that can be overridden in FxApplication to initialize the application at different stages when loading.
• Controller abstract class introduced with similar startup methods (initialize and beforeStart).
• Adding Light and Dark Theme.
• Adding About menu that displays the information from About.properties file. It also informs about the installed Open XAL version.
• Several TreeView widgets: AcceleratorTreeView, ComboSequencesTreeView, and PowerSuppliesTreeView.
• Fixing bugs related to AcceleratorProperty and SequenceProperty when not defined correctly (or not used).
• Fixed XMLDataManager methods to save accelerator optics files. Added as a new property to the application.
• Refactored Menu event handlers to make use of lambda functions.
• Updated to Java FX 16.
• Refactored readback handles methods in AcceleratorNode. Now it uses annotations on the channel handlers and reflections.
• New dipole bucket to simplify XML files. Each magnet type gets its own attributes.

This MR is NOT backwards compatible with older Open XAL versions in 2 aspects:

• Old lattice files won't work correctly.
• Old FX applications won't work.