feat: Allow check of configuration from an instance of ConfigurationModel, without any file involved.

feat: Read Minecraft, Forge and Fabric versions from their manifests into lists which can then be used in GUIs.

refactor: Prevent going through a list of clientside-only mods automatically gathered from modpack is property is false.

test: Remove addon execution from tests, as parallel running tests caused problems because the addon can only be accessed by one thread at a time.

refactor: Change labels for Minecraft, modloader and modloader version to better reflect new feature of selection from lists.

refactor: Move assignemts to field declaration where applicable. Extract method for adding MouseListeners to buttons.

fix: Correctlry get property which decides whether autodiscovery of clientside-only mods should be enabled

docs: Name correct filename for properties according to merge of lang.properties with serverpackcreator.properties

feat: New theme and cleaned up GUI. MenuBar for various things (wip). Lists for version selection. Switch between darkmode and lightmode and remember last mode used. More things, check commit.