Fixed lag issues, fully implemented tick system

- lag was coming from a method that loaded the texture for the NPC's equipped item (to render in its hand); the method was being constantly called
- also improved world generation speeds by using LinkedLists instead of ArrayLists (the former is more efficient when adding and removing objects to the list, like tiles)
- GameTick is now added everywhere Timers used to be (ex instead of doing something every 1000ms, it executes an action every 16 ticks instead)