[B+C] Add FallingBlockDeathEvent. Fixes BUKKIT-4160 and BUKKIT-4402

The Issue:

The plugin developer were not able to check if a FallingBlock died.

Justification for this PR:

By accepting this PR plugin developer would be able to check if a FallingBlock died. This can be really useful if you want a certain thing to happen right after a FallingBlock gets destroyed.

PR Breakdown:

The PR does exactly what it sounds like. It'll make the EntityFallingBlock fire a FallingBlockDeathEvent right before it gets destroyed. You may even cancel its death.

Testing Results and Materials:

To test the new event I temporarily added a line to the FallingBlockDeathEvent constructor that looked like this:

Bukkit.broadcastMessage("A block died :,(");

With this line added I compiled my modified CraftBukkit version, placed a sand block in the air and checked if the message appeared after the block lands. It did.

Relevant PR(s):

https://github.com/Bukkit/CraftBukkit/pull/1392 - second PR

JIRA Ticket:

BUKKIT-4402 - https://bukkit.atlassian.net/browse/BUKKIT-4402
BUKKIT-4160 - https://bukkit.atlassian.net/browse/BUKKIT-4160

Additional information:

I'm really sorry for making that many PRs but I already removed my original branch and reforked Bukkit/CraftBukkit to rewrite my code from scratch to fit the guidelines. so I CAN'T update the others PRs anymore.. I'm sorry for that and it won't happen again.
Should extend EntityEvent rather than Event.