[B+C] Add the method getFireBlock to BlockBurnEvent. Fixes BUKKIT-4231

Add a method to get the source (fire) block from the BlockBurnEvent.

Allows for more control for the fire (count the block is has burnt, for example, or put it out).

PR Breakdown:
Add a Block to the BlockBurnEvent, to store the fire block, and change the constructor to also get the fire block.

CB-1159 - https://github.com/Bukkit/CraftBukkit/pull/1159 - Implementation

BUKKIT-4231 - https://bukkit.atlassian.net/browse/BUKKIT-4231