CHANGED: reworked LoadingDialog,

 * use smaller invokeLater()s in mainGUI()
   => smoother startup
CHANGED: minor changes to support splash screen

 * rewrote doneCheckLoad()
 * enable launchButton only if authlib is ready to use
   and launcher installation directory is writeable
CHANGED: minor changes

 * show() LaunchFrame after pack info are loaded
 * minor LoadinDialog message changes