Cleaned up Initialization for GatewayGenerator

Cleaned up the code for initializing GatewayGenerator. We had some
static fields that didn't need to be static and we could just initialize
gateways from the class's constructor.
Added DeathTracker and Removed PlayerTracker

Added DeathTracker - a class for tracking the deaths of players on the
server while in pocket dimensions. It'll be used for generating skulls
for grave chests. Also removed PlayerTracker since it wasn't being used
for anything at all. I'm guessing it was supposed to be used for
something later. We can restore it if it's ever needed.
Minor Change

Minor change to spacing in PocketManager
Changes to EventHookContainer

Added code to save DeathTracker data when then Overworld saves, but only
if it's been modified since the last time it was saved. Also changed how
we handle player deaths in pocket dimensions. Now we have two death
handlers. There is a high-priority one that sends players to Limbo only
when Limbo preserves their inventories. The other is a low-priority
handler that sends players to Limbo only when inventory preservation is
disabled. This gives players a chance to keep their inventories if
another mod would have revived them without that penalty.
Changes to Loot and Skulls

There is 5% chance that grave chests will contain the head of a random
player that has died in a pocket dimension; there is a 20% chance of it
being a special head. Also moved Ghast Tears to grave chests and removed
Eyes of Enders as loot. Also made FoR loot more rare but increased the
amount given out so that the average amount picked up should be the same
as before. World Thread is slightly less common in response to being
easier to farm from rifts.