AppService: add method to get foremost app

The new getApp() method returns the first app on the list -- i.e., the
one with the highest priority.

Author: ctrueden

src/main/java/org/scijava/app/AppService.java

@@ -42,6 +42,9 @@
  */
 public interface AppService extends SingletonService<App> {
 
+	/** Gets the foremost application (the one with the highest priority). */
+	App getApp();
+
 	/** Gets an application by name. */
 	App getApp(final String name);
 

src/main/java/org/scijava/app/DefaultAppService.java

@@ -33,6 +33,7 @@
 
 import java.util.Collections;
 import java.util.HashMap;
+import java.util.List;
 import java.util.Map;
 
 import org.scijava.log.LogService;
@@ -61,6 +62,13 @@ public class DefaultAppService extends AbstractSingletonService<App> implements
 
 	// -- AppService methods --
 
+	@Override
+	public App getApp() {
+		final List<App> appList = getInstances();
+		if (appList == null || appList.isEmpty()) return null;
+		return appList.get(0);
+	}
+
 	@Override
 	public App getApp(final String name) {
 		return apps().get(name);