Automatically dispose the context on JVM shutdown

And make the ThreadService's threads into daemon threads.

The danger is that the such threads won't complete pending work. But
with the JVM shutdown hook now attempting to dispose the context at
that point, the ExecutorService will have a chance to shutdown cleanly.

Author: ctrueden

src/main/java/org/scijava/Context.java

@@ -290,6 +290,9 @@ public Context(final Collection<Class<? extends Service>> serviceClasses,
 				new ServiceHelper(this, serviceClasses, strict);
 			serviceHelper.loadServices();
 		}
+
+		// If JVM shuts down with context still active, clean up after ourselves.
+		Runtime.getRuntime().addShutdownHook(new Thread(() -> doDispose(false)));
 	}
 
 	// -- Context methods --

src/main/java/org/scijava/thread/DefaultThreadService.java

@@ -173,7 +173,14 @@ public synchronized void dispose() {
 	@Override
 	public Thread newThread(final Runnable r) {
 		final String threadName = contextThreadPrefix() + nextThread++;
-		return new Thread(r, threadName);
+		final Thread thread = new Thread(r, threadName);
+		// NB: Use daemon threads for the thread pool, so that idling threads do
+		// not prevent the JVM shutdown sequence from starting. The application
+		// context, and therefore the thread service, will try to dispose itself
+		// upon JVM shutdown, which will invoke executor.shutdown(), so there
+		// will be a chance for these threads to complete any pending work.
+		thread.setDaemon(true);
+		return thread;
 	}
 
 	// -- Helper methods --