Gateway: add a handy launch(String...) method

This performs launch operations standard to most SciJava applications.
This code migrated from a static method of imagej/imagej:

    net.imagej.Main.launch(String...)

With this change, the standard launch process is accessible to all
SciJava applications in a DRY way, while still being configurable
for applications with special needs. Typically, such applications will
define their own Gateway implementation which extends AbstractGateway;
hence, these applications can override launch(String...) to modify
their launch behavior.

We considered creating a LaunchService with LaunchAction plugins,
but it seemed potentially overcomplicated.

Author: ctrueden

pom.xml

@@ -10,7 +10,7 @@
 	</parent>
 
 	<artifactId>scijava-common</artifactId>
-	<version>2.52.2-SNAPSHOT</version>
+	<version>2.53.0-SNAPSHOT</version>
 
 	<name>SciJava Common</name>
 	<description>SciJava Common is a shared library for SciJava software. It provides a plugin framework, with an extensible mechanism for service discovery, backed by its own annotation processor, so that plugins can be loaded dynamically. It is used by both ImageJ and SCIFIO.</description>

src/main/java/org/scijava/AbstractGateway.java

@@ -89,6 +89,23 @@ public AbstractGateway(final String appName, final Context context) {
 
 	// -- Gateway methods --
 
+	@Override
+	public void launch(final String... args) {
+		// parse command line arguments
+		console().processArgs(args);
+
+		// launch main methods
+		final int mainCount = main().execMains();
+
+		// display the user interface (NB: does not block)
+		if (mainCount == 0 && !ui().isHeadless()) ui().showUI();
+
+		if (ui().isHeadless()) {
+			// now that CLI processing/execution is done, we can shut down
+			getContext().dispose();
+		}
+	}
+
 	@Override
 	public String getShortName() {
 		return getClass().getName().toLowerCase();

src/main/java/org/scijava/Gateway.java

@@ -120,6 +120,24 @@
  */
 public interface Gateway extends RichPlugin, Versioned {
 
+	/**
+	 * Perform launch operations associated with this gateway.
+	 * <p>
+	 * Typical operations might include:
+	 * </p>
+	 * <ul>
+	 * <li>Handle the given command line arguments using the
+	 * {@link ConsoleService}.</li>
+	 * <li>Execute registered main classes of the {@link MainService}.</li>
+	 * <li>Display the default user interface using the {@link UIService}.</li>
+	 * <li>In some circumstances (e.g., when running headless), dispose the
+	 * context after launch operations are complete.</li>
+	 * </ul>
+	 * 
+	 * @param args The arguments to pass to the application.
+	 */
+	void launch(String... args);
+
 	/**
 	 * Gets a very succinct name for use referring to this gateway, e.g. as a
 	 * variable name for scripting.