Tray-API: Add method to update tray icon #13

Updated javadoc as requested.

Author: VsevolodGolovanov

src/main/java/org/cryptomator/integrations/tray/TrayMenuController.java

@@ -31,9 +31,10 @@ static Optional<TrayMenuController> get() {
 	void showTrayIcon(byte[] imageData, Runnable defaultAction, String tooltip) throws TrayMenuException;
 
 	/**
-	 * Updates an icon on the system tray.
+	 * Updates the icon on the system tray.
 	 *
 	 * @param imageData What image to show
+	 * @throws IllegalStateException thrown when called before an icon has been added
 	 */
 	void updateTrayIcon(byte[] imageData);