Improvements/bug fixes, based both on user feedback and things I thought could be better

Bugs:

• Visual bug in game settings: When selecting control options and exiting to the menu, the "left"/"right" arrows would become visually active.
  
• Bug in the "play" button on the main menu: If the player had already completed the entire game and used the "play" button, the game would crash on a dark screen.
  

Improvements

• UI: You can now toggle the User Interface on/off, no longer needing to hold the button down to see current skills.
  
• Added two more songs that will be played during gameplay, I noticed that many users have more than two hours of gameplay and I thought listening to the same song for a long time would be a little annoying. Credits added to the main menu.
  
• Added new achievement: Killer: earned by killing Oliver with Earthquake. (For those who have already played the game and are only interested in having all the achievements: it can be easily obtained in level 7)