Updates

- Added new intro to the first level to get some simple mechanics out of the way.
- Added new battlefield before the first level battle-arena to demonstrate doors unseal when all foes are defeated.
 - Added quick spawn-in sequence.
 - Added zoom into door.
 - Modified difficulty names.
 - Added a first switch to the second level to remind the player of switches.
 - Added a notice in the second level that you not only need to unlock the switch, you need to shoot it as well.