Adding a gradle task to build Sparkle Motion into a Mac app.

Ideally it would come up as a normal-looking Mac application, but lwjgl and Swing aren't compatible because they both want full control of the main thread's event loop. So unfortunately it starts the app with a taskbar icon but no menu bar or windows.

See also:

• What does the -XstartOnFirstThread VM Argument do / mean?
• jpackage-gradle-plugin for the official java packaging wrapper
• lwjgl3-awt, but it apparently doesn't work on Mac