Visual overhaul of the game. Added button images, fonts and improved game's GUI.

[amrit-choudhary]

Fixed: mouse scroll as click Missing:

• Some elements in game_over_screen are not done yet.
• Game modes are missing. Click on any difficulty button to start the game.

Screenshots:

[vineetjc]

@amrit-choudhary check below:

• Explain Hardcore and Heist to me.
• With regards to the above, should we keep some of the buttons away for now and bring it as and when the game mode is introduced? For now, if I'm merging it I don't mind, I will keep it. Need opinions though.
• Score and Highscore texts aren't drawn correctly, I think it's a little off towards the top, and so is the restart button, a little too high away from the back button, please check if this is also your issue.

I'll wait for the changes, otherwise, I'm already very impressed by the visuals!

[amrit-choudhary]

@vineetjc

• They are just placeholder buttons, which are not active now. The actual game modes can be different. I wanted to test how it will look with all the modes active in the final game. I can remove them if you want.
• It's your choice.
• I will move score some pixels down. The game_over_screen is still in development and is not final. Please ignore that.