Started Main Menu

- Implemented Main Menu JPanel
- Implemented non-functional "Play" button
- Implemented Main Menu being shown after launch
- Implemented "Critical Mass"-Headline being displayed   on main menu
- Added pictures menu/button, menu/button_highlighted, menu/play,
menu/options, menu/exitgame, menu/headline in /assets/png/

(This is commit #002 to version A1.0)