I fixed a few bugs and crashes from the last demo.

• The main menu option in the difficulty selection screen has been changed to "back" and will take you to the previous screen instead of always the main menu which was inconvenient if you were doing a custom game.
• The power attack word will reset properly after each round. It was preventing the user to write the same word again when the panel showed up.
• Fixed a crash when using the power-attack against the sorcerer boss.
• Fixed the knight not finishing the attack if writing a word while being attacked by a ghost.
• Fixed using power-attacks against the ghost.
• Fixed a crash caused by the chains. (Couldn't reproduce this one, it might still be there.)