Fix for broken contract for GroupNode node comparator.

Please verify this implementation as I haven't been able to reproduce the exact state for testing!

This fixes a slight implementation error for the Comparator. I noticed the error because Java's sorting algorithm (TimSort) threw an IllegalArgumentException. The following change should fix that. Error information following.

This comparator's general contract is broken for the case where both
node1 and node2 have negative source indexes. In that case the returned
results are:
- (node1, node2) => -1
- (node2, node1) => -1
which is not symmetric, as is expected by the Comparator interface.

This is noticed by the TimSort implementation when sorting:
12:59:34.108 SEVERE: [38] util.UtilActivator.uncaughtException().108 An uncaught exception occurred in thread=Thread[AWT-EventQueue-0,6,main] and message was: Comparison method violates its general contract!  java.lang.IllegalArgumentException: Comparison method violates its general contract!
	at java.util.TimSort.mergeHi(TimSort.java:868)
	at java.util.TimSort.mergeAt(TimSort.java:485)
	at java.util.TimSort.mergeCollapse(TimSort.java:410)
	at java.util.TimSort.sort(TimSort.java:214)
	at java.util.TimSort.sort(TimSort.java:173)
	at java.util.Arrays.sort(Arrays.java:659)
	at java.util.Collections.sort(Collections.java:217)
	at net.java.sip.communicator.impl.gui.main.contactlist.GroupNode$1.run(GroupNode.java:323)
	at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:312)
	at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:733)
	at java.awt.EventQueue.access$200(EventQueue.java:103)
	at java.awt.EventQueue$3.run(EventQueue.java:694)
	at java.awt.EventQueue$3.run(EventQueue.java:692)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
	at java.awt.EventQueue.dispatchEvent(EventQueue.java:703)
	at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:242)
	at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:161)
	at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:150)
	at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:146)
	at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:138)
	at java.awt.EventDispatchThread.run(EventDispatchThread.java:91)

This proposition tries to distinguish between two nodes by comparing
their hashCode value. This is kind of a work around, however, since this
is already a special case it should not matter that much. Since the
hashCode is a comparable value we can ensure that we make a consistent,
symmetric choice every time. Thus enforcing the contract.