Allow switching blocks when blocks moving

[cxong]

Not allowing block switching when blocks moving causes a large delay when blocks are falling many layers. Removing this limitation means that player control is consistent regardless of what's happening in the game.

Demonstration with slowed block speed:

[dorkster]

I just pushed https://github.com/dorkster/freeblocks/commit/e473a579ca0f0469184a319f50a907198ce37fba, which is basically the changes you submitted, but with a few tweaks so that the player can't switch blocks that are already moving.