2.7 Fixing up the demo

I got a lot of feedback on the initial demo, that basically fit into four categories:

• Bugs (Castling when not allowed and not identifying checkmates in rare circumstances).
• People complaining that the opening moves were too fast.
• Difficulty (Mostly people saying it's too hard, some people saying it's too easy).
• Complaints that it isn't chess.

I fixed up the bugs, was a quirk with how the undo button was affecting the state of the game.
I added a speed slider so players can set their own opening move speeds.
I added some easier levels to the start to help those that found it too hard.
The final one is a good example of why you shouldn't take criticism at face value.