[B+C] Implement Dropper interface. Adds BUKKIT-3750

The Issue:

Actually , there is no way to get the Dropper inventory with the simple Bukkit API , and you can't cast the BlockDispenseEvent block.getState() to a Dispenser cause that return a CraftBlockState

Justification for this PR:

This PR add an Interface for Dropper that create a correct CraftDropper extends CraftBlockState

PR Breakdown:

This PR implement the Dropper interface with create is own CraftBlockState like with a Dispenser

Relevant PR:

Bukkit: https://github.com/Bukkit/Bukkit/pull/806 

Jira Ticket :
Leaky: https://bukkit.atlassian.net/browse/BUKKIT-3750