Added B-Mode

I added the B-Mode in which you have to clear 25 lines to win. Depending on the level you select there are "noise blocks" already on the screen when you start.

As far as I tested it, everything should work fine and does not interfere with the old A-Mode. I've not added any ranking-related code though.

Unfortunately I could not remember the exact rules of the B-Mode. I now added 3 lines of noise for every additional level (so its 0 to 12 lines). And currently there is a chance of 50% that a field is a block - I've added this as a constant so it's easily changable.

If there is something missing or wrong, please comment, so I can add / change it before you merge this version.