Make single-instance behavior opt-in via scijava.app.single-instance property

Single-instance mode is now only activated when the system property
scijava.app.single-instance=true is set. This makes the feature opt-in
rather than mandatory, allowing applications that need multiple
concurrent instances to run without modification.

Author: ctrueden

src/main/java/org/scijava/launcher/ClassLauncher.java

@@ -80,7 +80,9 @@ public static void main(final String... args) {
 		if (Boolean.getBoolean("scijava.app.unlock-modules")) {
 			ReflectionUnlocker.unlockAll();
 		}
-		if (SingleInstance.tryHandoff(args)) System.exit(0);
+		if (Boolean.getBoolean("scijava.app.single-instance")) {
+			if (SingleInstance.tryHandoff(args)) System.exit(0);
+		}
 		tryToRun(Splash::show);
 		tryToRun(Java::check);
 		String appName = appName();