hookify software_entity_launcher and single_config_launcher

[kramester]

I converted the BaseLauncher class into a base hook and made the subclasses SingleConfigLauncher and SoftwareEntityLaunchers into hooks. This way a developer can completely replace the logic here with their own launcher code. The app remains intact and works the exact same as before but this deprecates the need for the use_software_entity bool setting and instead uses a hook_launcher hook setting to determine which launcher to use.