ADD: DefaultLegacyOpener can open SelfShowableContent...

...which is a new kind of result of opening of inputs, often
used in conjuction with openers that do load-and-show on their own,
as opposite to load-only-and-have-Fiji-to-show pattern.

src/main/java/net/imagej/legacy/plugin/DefaultLegacyOpener.java

@@ -63,6 +63,7 @@
 import org.scijava.plugin.Plugin;
 import org.scijava.plugin.PluginService;
 import org.scijava.service.Service;
+import org.scijava.ui.SelfShowableContent;
 
 /**
  * The default {@link LegacyOpener} plugin.
@@ -200,6 +201,10 @@ public Object open(String path, final int planeIndex,
 	private Object handleData(Context c, Object data, String path, boolean displayResult) {
 		if (data == null) return path;
 
+		if (displayResult && data instanceof SelfShowableContent) {
+			((SelfShowableContent)data).show();
+			return data;
+		}
 		if (data instanceof Dataset) {
 			final Dataset d = (Dataset) data;
 			ImagePlus imp = null;