Added .gitignore; cleaned up JetBrains files and .class files

.gitignore

HelloWorld/.idea/.gitignore

HelloWorld/.idea/compiler.xml

HelloWorld/.idea/description.html

HelloWorld/.idea/encodings.xml

HelloWorld/.idea/gradle.xml

HelloWorld/.idea/misc.xml

HelloWorld/.idea/modules.xml

HelloWorld/.idea/uiDesigner.xml

HelloWorld/.idea/vcs.xml

HelloWorld/HelloWorld.iml

...t/production/HelloWorld/META-INF/HelloWorld.kotlin_module

...oWorld/out/production/HelloWorld/app/HomeController.class

HelloWorld/out/production/HelloWorld/app/Main.class

...ut/production/HelloWorld/app/SecondScreenController.class

HelloWorld/out/production/HelloWorld/app/home.fxml

HelloWorld/out/production/HelloWorld/app/secondScreen.fxml