Quality of life and performance improvements

Game Engine has been changed from XNA to MonoGame, and while doing this, the following improvements were made:

• Performance Improvements overall.
• Different colors in some levels (better visibility).
• Background elements added to some levels.
• Weapons are now shown as sprites instead of jewels.
• Overall difficulty reduced.
• Weapons do not run out of ammo.
• Weapons are kept upon death.
• Two sub-par levels removed from campaign.
• Outdated menu item removed.