TASK-6: add default UI skin from LibGDX
TASK-6: create screen management infrastructure

- refractor Game to ScorchedPlanet
  - remove proof of concept code
  - set up global skin (use default skin from test project for now)
  - launch MainMenuScreen
- create BaseScreen
  - keeps reference to the Game object
  - implement the Screen interface
  - create Stage and draw on render() method
- create MainMenuScreen
  - extends BaseScreen
  - add Label and TextField to Stage in show() method
- create LevelScreen
  - extends BaseScreen