[Bleeding] Implement new BlockState for flower pots. Adds BUKKIT-5316

In Minecraft 1.7, flower pots changed to store their data in a tile entity
due to the addition of new plants. This allows for getting and setting the
contents of flower pots via a new BlockState implementation, CraftFlowerPot.
[Bleeding] Update flower pots when interactions are cancelled. Addresses BUKKIT-5316

Flower pots are handled client-side, so we need to send an update packet to
the client if we cancel a PlayerInteractEvent and deny a flower from being
placed in the pot.