Move container block contents when container block is moved

[cromo]

Container blocks (see #15) are a natural way to group blocks under another block, and it would also be a natural way to move all the blocks contained in it. This proposes that when a container block is moved, the chain of all contents blocks should move and rotate relative to the container block. This will provide a way to move multiple blocks, instead of having to move every block one at a time.