Minor Stuff

The changes are:

• The green frame in the credits no longer disappears when closed.
• Correction of grammar error in "94. Mirrored Translation".
• The perdiodic translation modifiers stack up correctly, now. This fixes some wrong behavior in "21. Movement is Relative".
• Refiddling of some parameters in "24. Up And Down". The platform is now less likely to slide away from under the player. 
• Change to the way the maximal falling velocity is enforced. This avoids odd behavior when jumping into a frame, while the outgoing side is falling and smaller than the ingoing frame side.
• Due to high demand (by which I mean two people who actually asked for something else) there is now a "sandbox level" available. Sandbox here just means a wide, open level with many objects in it and no goal. Tbh, I'm not sure how much this adds to the game, but maybe someone has some use for it.