Small completionism improvement

Some players were rightly confused by the optional tags and the amount of crystals in the game, here's something to make completionism more straightforward.

• Replaced the skull tag with a dead demon tag. It should be clearer now that you get that by killing every demon in the room.
  
• Made the demon tag red instead of blue. Red is clearly associated with the enemies, it makes more sense.
  
• Removed the heart tag. The game is already challenging as it is, and while it's nice to keep track of the levels where you managed not to get hit a single time the information wasn't used anywhere, and adding an achievement for finishing the game untouched would just make everyone salty. Get hit at your heart's content!
  
• Added easy crystals to normal rooms that were missing one. Just for consistency, they don't add challenge but it was confusing to find some random levels without a gem.