Fix fat JAR generation

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

pom.xml

@@ -61,23 +61,22 @@
 		<plugins>
 			<plugin>
 				<groupId>org.apache.maven.plugins</groupId>
-				<artifactId>maven-assembly-plugin</artifactId>
-				<version>3.2.0</version>
+				<artifactId>maven-shade-plugin</artifactId>
+				<version>3.2.4</version>
 				<executions>
 					<execution>
 						<phase>package</phase>
 						<goals>
-							<goal>single</goal>
+							<goal>shade</goal>
 						</goals>
 						<configuration>
-							<archive>
-								<manifest>
-									<mainClass>envoy.client.ui.Startup</mainClass>
-								</manifest>
-							</archive>
-							<descriptorRefs>
-								<descriptorRef>jar-with-dependencies</descriptorRef>
-							</descriptorRefs>
+							<shadedArtifactAttached>true</shadedArtifactAttached>
+							<sharedClassifierName>envoy</sharedClassifierName>
+							<transformers>
+								<transformer implementation="org.apache.maven.plugins.shade.resource.ManifestResourceTransformer">
+									<mainClass>envoy.client.Main</mainClass>
+								</transformer>
+							</transformers>
 						</configuration>
 					</execution>
 				</executions>

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); }
 }