Integrated engine. Recoded parts of the main menu. Added "play" button functionality

- integrated LWJGL and Slick2D as our engines of choice.
- Added a border and a close button to the main menu.
- Made the main menu window undecorated.
- Clicking the "play" button now launches a singleplayer game.
- Added the core class for the singleplayer mode

(this is commit #004 to version 1.0A)