Added new levels and game-over screen

* LoadingScreen.java
- removed drawing of text "Loading"
- removed unloading of loading screen's background image
- added loading of game-over background and level 2 & 3 pixmaps

* BobController.java
- added showGameOverScreen() method
- added method to change level

* GameOverScreen.java
- render player's score on game over
- return to main menu when back key is pressed, screen is touched or
enter key is pressed

* MonsterAssault.java
- added new constant to store no. of levels present in the game

* Block.java
- modified setType() method to update the block texture to new block
type
- made getTexture(Type blockType) method static

* Level.java
- added attributes to store door block and no. of enemies in level
- added corresponding getters and setters

* LevelLoader.java
- set the door block in level
- set number of enemies present in the level

* World.java
- added static attribute to store no. of enemies left in the World
- modified constructor to accept the level no. to create world for
- modified removeEnemy() method to decrement the no. of enemies left in
the world
- added method clear() to reset the static fields of the class so that
previous objects are not kept when game screen is loaded again

* GameScreen.java
- added attribute to store current level no. and corresponding getters
and setters
- modified call to World constructor to include level no. to be created
- modified render method to change the state of door block when all
enemies are dead

* New assets:
- background image for Game-over Screen
- level 2
- level 3
- modified loading screen background