Avoid displaying welcome message when running headless

See the discussion at http://forum.imagej.net/t/running-fiji-in-headless-mode-inside-docker-container/7626

Author: imagejan

src/main/java/org/scijava/welcome/DefaultWelcomeService.java

@@ -47,6 +47,7 @@
 import org.scijava.service.AbstractService;
 import org.scijava.service.Service;
 import org.scijava.text.TextService;
+import org.scijava.ui.UIService;
 import org.scijava.ui.event.UIShownEvent;
 import org.scijava.util.DigestUtils;
 import org.scijava.welcome.event.WelcomeEvent;
@@ -83,6 +84,9 @@ public class DefaultWelcomeService extends AbstractService implements
 	@Parameter
 	private EventService eventService;
 
+	@Parameter
+	private UIService uiService;
+
 	@Parameter
 	private PrefService prefService;
 
@@ -127,7 +131,7 @@ public void setFirstRun(final boolean firstRun) {
 	/** Displays the welcome text when a UI is shown for the first time. */
 	@EventHandler
 	protected void onEvent(@SuppressWarnings("unused") final UIShownEvent evt) {
-		if (!isFirstRun()) return;
+		if (uiService.isHeadless() || !isFirstRun()) return;
 		eventService.publish(new WelcomeEvent());
 		setFirstRun(false);
 		displayWelcome(false);