MainGui: set icon for Taskbar

Use java.awt.Taskbar added in Java 9 to set the taskbar icon image. This
mainly affects macOS, as far as I know, which doesn't automatically take
the icon of the JFrame (`window`).

The default Duke icon will be replaced by the Resoday logo in the macOS
dock.  The downside is that macOS doesn't segregate different Java
applications, so all apps running under with the same binary file `java`
are gathered under the one icon.

CHANGELOG

@@ -1,5 +1,7 @@
 - Rendering of the icon in the title bar of the main window has been
   fixed in Microsoft Windows.
+- Dock on macOS will now show the Resoday icon instead of the default
+  Duke icon of the JVM.
 
 v1.5
 - Fixed dead link in "About" dialog

src/main/java/dev/andrybak/resoday/gui/Logo.java

@@ -26,6 +26,12 @@ public static List<Image> getImages() {
 		);
 	}
 
+	public static Image getMultiResolutionImage() {
+		Image icon32 = getImage(APP_ICON_32_FILENAME);
+		Image icon64 = getFixedResolutionImage();
+		return new BaseMultiResolutionImage(icon32, icon64);
+	}
+
 	private static Image getImage(String filename) {
 		URL url = Objects.requireNonNull(MainGui.class.getResource(filename));
 		return Toolkit.getDefaultToolkit().getImage(url);

src/main/java/dev/andrybak/resoday/gui/MainGui.java

@@ -35,6 +35,7 @@
 import java.awt.BorderLayout;
 import java.awt.Desktop;
 import java.awt.Dimension;
+import java.awt.Taskbar;
 import java.awt.event.ActionEvent;
 import java.awt.event.InputEvent;
 import java.awt.event.KeyEvent;
@@ -388,6 +389,9 @@ private void goInEdt(Path configDir) {
 		 * Can't use Logo.getMultiResolutionImage() here because of a JDK bug on Linux.
 		 */
 		window.setIconImages(Logo.getImages());
+		if (Taskbar.isTaskbarSupported() && Taskbar.getTaskbar().isSupported(Taskbar.Feature.ICON_IMAGE)) {
+			Taskbar.getTaskbar().setIconImage(Logo.getMultiResolutionImage());
+		}
 		window.setVisible(true);
 		window.addWindowListener(new WindowAdapter() {
 			@Override