KeeperRL Alpha 31 hotfix 3

This patch adds the following content:

• The Halloween Zombies faction is now hidden by default and can be unlocked through a secret action in the game.
  
• Another novelty playable faction has been added and has to be unlocked.
  

And fixes:

• The game won't allow playing as retired villains (enemies that have conquered your base and are now living in it) in the warlord mode.
  
• Fixed a glitch in the tutorial build menu.
  
• Fixed a rare UI crash when clicking on a minion in the minion menu.