Add flower pot implementation

For this pull request is this pull request for Glowkit required: https://github.com/GlowstoneMC/Glowkit/pull/48. This basically adds all the necessary implementation for it.

**Modifications**
Add the implementation, as block state, for `org.bukkit.block.FlowerPot` introduced in the previously mentioned pull request. This is the only class that distinguishes the pre-1.7 way of handling flower pot contents (block data) and the post-1.7 way (tile entities). This implementation is also now registered to the item table. Apart from that a flower pot tile entity and block type implementation are added.

**Result**
Proper past, present and future proof handling of flower pots.
I saw it, but didn't find it fitting to include in this pull request. I personally think it would be much more practical to write a separate pull request for updating relevant tile entities to easily maintain consistency. And yes, you're correct, it is for visual updates mostly, yet `TEMobSpawner` among one of the tile entities that require visual updates, hasn't overridden `TileEntity#update(GlowPlayer)`...