[B+C] Add a way to get and set flower pot contents. Fixes BUKKIT-5316

##### The Issue:
With new flowers in Minecraft 1.7, Mojang changed the way content of flower pot is saved. This made it impossible to get and set flower pot content correctly.

##### Justification:
System that exist right now (org.bukkit.material.FlowerPot) is outdated and it works only for versions o Minecraft lower than Minecraft 1.7. Flower pot contents is now saved using NBT data.

##### PR Breakdown:
This PR adds a Flower Pot Block State interface that allows plugins to get and set flower pot contents.

##### Bukkit PR:
[Bukkit/CraftBukkit#1333](https://github.com/Bukkit/CraftBukkit/pull/1333)

##### JIRA Ticket: 
[JIRA Ticket #5368](https://bukkit.atlassian.net/browse/BUKKIT-5368) (Duplicate of [JIRA Ticket #5316](https://bukkit.atlassian.net/browse/BUKKIT-5316))