[B] Adds method PlayerDeathEvent.getPlayer() - Adds BUKKIT-4885

The Issue:
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PlayerDeathEvent may be thought to not have a getPlayer method and the getEntity method may be confused as something that does not in fact return the Player involved in the event.

Justification for this PR:
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This commit deprecates the somewhat confusing and inaccurately named method PlayerDeathEvent.getEntity() and replaces it with a more accurately names PlayerDeathEvent.getPlayer() as this method always returns a player so it should be named getPlayer() to reduce confusion.

PR Breakdown:
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getEntity() - deprecated so still possible for un updated code and to help reduce confusion
getPlayer() - new method to replace getEntity to cause lest confusion and a more accurately named method as the getEntity method always returns a player.

Testing Results and Materials:
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Testing using a arbitrary bukkit plugin to ensure that the code was working.

JIRA Tickets:
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https://bukkit.atlassian.net/browse/BUKKIT-4885