Remove all remaining entities when closing the event
Add player-leave trigger, check player/team event and auto end event

* Add new built in trigger player-leave which triggers when a player dies or leaves the arena. The built in trigger player-death is present and only triggers on a players death.
* Check team/player event is be used to check if the specified number of players/teams or less are active in the arena and trigger a one shot event if they drop below the limit.
* Events can now be auto ended by signalling end-event which causes the arena to be ended.