UIService: add headless methods

Add isHeadless and setHeadless methods to the UIService. These methods
delegate directly to the "java.awt.headless" system property.

Author: hinerm

src/main/java/org/scijava/ui/DefaultUIService.java

@@ -203,6 +203,17 @@ public boolean isVisible(final String name) {
 		return ui.isVisible();
 	}
 
+
+	@Override
+	public void setHeadless(final boolean headless) {
+		System.setProperty("java.awt.headless", String.valueOf(headless));
+	}
+
+	@Override
+	public boolean isHeadless() {
+		return Boolean.getBoolean("java.awt.headless");
+	}
+
 	@Override
 	public UserInterface getDefaultUI() {
 		if (defaultUI != null) return defaultUI;
@@ -527,5 +538,4 @@ private void addUserInterface(final String name, final UserInterface ui) {
 	private String getTitle() {
 		return appService.getApp().getTitle();
 	}
-
 }

src/main/java/org/scijava/ui/UIService.java

@@ -102,6 +102,12 @@ public interface UIService extends SciJavaService {
 	/** Gets whether the UI with the given name or class name is visible. */
 	boolean isVisible(String name);
 
+	/** Sets whether the application is running in headless mode (no UI). */
+	void setHeadless(boolean isHeadless);
+
+	/** Gets whether the UI is running in headless mode (no UI). */
+	boolean isHeadless();
+
 	/**
 	 * Gets the default user interface.
 	 *