Resolve "Refactor main menu with scene2d"

Changes in this merge request:

• All rendering code in MainMenuScreen has been alterned to use scene2d in the same way as is done for LoginScreen.
• MainMenuController has been altered to simply set the event listeners for each of the four buttons.
• Transitions to GameScreen, LeaderBoardScreen, and LoginScreen has been implemented.
• Changed the width and height fields in TankWarsGame to fit the menus' dimensions (landscape mode)
• The screen is now locked in landscape mode
• Some comments added and code clean-up done for LoginController and LoginScreen.

Closes #28 (closed)