Fix fat JAR generation

* Replace Maven Assembly Plugin with Maven Shade Plugin
* Extract main method to separate Main class

src/main/java/envoy/client/Main.java

@@ -0,0 +1,30 @@
+package envoy.client;
+
+import javafx.application.Application;
+
+import envoy.client.ui.Startup;
+
+/**
+ * Triggers application startup.
+ * <p>
+ * To allow Maven shading, the main method has to be separated from the
+ * {@link Startup} class which extends {@link Application}.
+ * <p>
+ * Project: <strong>envoy-client</strong><br>
+ * File: <strong>Main.java</strong><br>
+ * Created: <strong>05.07.2020</strong><br>
+ *
+ * @author Kai S. K. Engelbart
+ * @since Envoy Client v0.1-beta
+ */
+public class Main {
+
+	/**
+	 * Starts the application.
+	 *
+	 * @param args the command line arguments are processed by the
+	 *             client configuration
+	 * @since Envoy Client v0.1-beta
+	 */
+	public static void main(String[] args) { Application.launch(Startup.class, args); }
+}

src/main/java/envoy/client/ui/Startup.java

@@ -128,13 +128,4 @@ public final class Startup extends Application {
 			logger.log(Level.SEVERE, "Unable to save local files: ", e);
 		}
 	}
-
-	/**
-	 * Starts the application.
-	 *
-	 * @param args the command line arguments are processed by the
-	 *             {@link ClientConfig}
-	 * @since Envoy Client v0.1-beta
-	 */
-	public static void main(String[] args) { launch(args); }
 }