Replace crude self-made addon system with Pf4j and provide first entry points

Allow execution of addons before server pack generated, after, before archive creation and allow adding of new tabs in the GUI.

backend/main/java/de/griefed/serverpackcreator/ApplicationPlugins.java

+/* Copyright (C) 2022  Griefed
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301
+ * USA
+ *
+ * The full license can be found at https:github.com/Griefed/ServerPackCreator/blob/main/LICENSE
+ */
 package de.griefed.serverpackcreator;
 
 import de.griefed.serverpackcreator.plugins.serverpackhandler.PostGenExtension;
@@ -15,6 +34,12 @@ import java.io.File;
 import java.io.IOException;
 import java.util.List;
 
+/**
+ * Manager for ServerPackCreator plugins. In itself it doesn't do much. It gathers lists of all available extensions for
+ * {@link TabExtension},{@link PreGenExtension},{@link PreZipExtension} and {@link PostGenExtension} so they can then be
+ * run during server pack generation and during initialization of the GUI.
+ * @author Griefed
+ */
 @Component
 public class ApplicationPlugins extends JarPluginManager {