When a block placement happens we currently update physics on the
attempted placement and update again if the placement is cancelled.
To correct the first one we simply set the block without applying
physics. To correct the second we have to add a new method to
BlockState that lets us update without applying physics and use
this method method when putting the block back.
- Removed the useless world field.
- Made it so changes to a CraftSign (which is a Block*State*) no longer reflect into the world without calling sign.update().
