Fix Bug 840: DefaultEDTUtil.restart() shall only reuse ThreadGroup (tg) is not destroyed, otherwise use current thread's tg.

With jdk7u40, when re-launching a NEWT applet (JOGLNewtApplet1Run),
i.e. via browser back and forth, the following exception happens:

java.lang.RuntimeException: java.lang.IllegalThreadStateException
    at com.jogamp.newt.awt.applet.JOGLNewtApplet1Run.init(JOGLNewtApplet1Run.java:218)
    at com.sun.deploy.uitoolkit.impl.awt.AWTAppletAdapter.init(Unknown Source)
    at sun.plugin2.applet.Plugin2Manager$AppletExecutionRunnable.run(Unknown Source)
    at java.lang.Thread.run(Unknown Source)
Caused by: java.lang.IllegalThreadStateException
    at java.lang.ThreadGroup.addUnstarted(Unknown Source)
    at java.lang.Thread.init(Unknown Source)
    at java.lang.Thread.<init>(Unknown Source)
    at jogamp.newt.DefaultEDTUtil$NEDT.<init>(DefaultEDTUtil.java:280)
    at jogamp.newt.DefaultEDTUtil.restart(DefaultEDTUtil.java:91)
    at jogamp.newt.DisplayImpl.runOnEDTIfAvail(DisplayImpl.java:231)
    at jogamp.newt.WindowImpl.runOnEDTIfAvail(WindowImpl.java:1758)
    at jogamp.newt.WindowImpl.setUndecorated(WindowImpl.java:1477)
    at com.jogamp.newt.opengl.GLWindow.setUndecorated(GLWindow.java:278)
    at com.jogamp.newt.awt.applet.JOGLNewtApplet1Run.init(JOGLNewtApplet1Run.java:188)
    ... 3 more

This is due to 7u40's changed ThreadGroup (tg) lifecycle, i.e. the tg is destroyed.
In such case, DefaultEDTUtil.restart() shall use the current threads tg.