build: Ensure test-dir is used at all times. Various misc changes

.runConfigurations/Run App.run.xml

...main/kotlin/serverpackcreator.java-conventions.gradle.kts

serverpackcreator-api/src/main/resources/CHANGELOG.md

...eator-api/src/test/resources/serverpackcreator.properties

serverpackcreator-app/jpackagerResources/app.icns

serverpackcreator-app/jpackagerResources/app.ico

serverpackcreator-app/jpackagerResources/app.png

serverpackcreator-app/jpackagerResources/appimage.desktop

serverpackcreator-app/jpackagerResources/appimage.png

serverpackcreator-app/jpackagerResources/appimage.svg

serverpackcreator-help/Writerside/topics/CHANGELOG.md