Add .gitignore

See merge request !1

Deleted .DS_Store, AppOpener.iml, .idea/libraries/Maven__org_openjfx_javafx_base_16.xml, .idea/libraries/Maven__org_openjfx_javafx_base_mac_16.xml, .idea/libraries/Maven__org_openjfx_javafx_controls_16.xml, .idea/libraries/Maven__org_openjfx_javafx_controls_mac_16.xml, .idea/libraries/Maven__org_openjfx_javafx_graphics_16.xml, .idea/libraries/Maven__org_openjfx_javafx_graphics_mac_16.xml, .idea/libraries, .idea/compiler.xml, .idea/encodings.xml, .idea/misc.xml, .idea/modules.xml, .idea/uiDesigner.xml, .idea/workspace.xml, .idea files