DefaultLegacyHooks: also skip closed ImageWindows

When ImageJ1 is quitting, the WindowManager.closeAllWindows() method
marks each ImageWindow as closed, but does not actually hide it. So the
ImageJ2 heuristic of checking whether the window is still visible after
it receives a windowClosing event fails. The workaround is to check, in
the case of ImageWindows, whether their isClosed() method returns true,
and if so, continue.

This change results in ImageJ1 shutting down successfully again when
image windows are still open.

src/main/java/net/imagej/legacy/DefaultLegacyHooks.java

@@ -32,6 +32,7 @@
 package net.imagej.legacy;
 
 import ij.ImagePlus;
+import ij.gui.ImageWindow;
 
 import java.awt.Window;
 import java.awt.event.KeyEvent;
@@ -560,7 +561,13 @@ public boolean disposing() {
 	// -- Helper methods --
 
 	private boolean isOpenWindow(final Window window) {
-		return window.isVisible();
+		return window.isVisible() && !isClosedImageWindow(window);
+	}
+
+	private boolean isClosedImageWindow(final Window window) {
+		if (!(window instanceof ImageWindow)) return false;
+		final ImageWindow imageWindow = (ImageWindow) window;
+		return imageWindow.isClosed();
 	}
 
 }