Bug fixes, UI tweaks, upgraded potions and more

- fixed not being able to take from chests
- storage unit GUI highlights the slot your mouse hovers over
- moved ammo count to side of screen
- added quiver (still a work in progress)
- fixed explosion particles not showing when shooting an enemy
- fixed non-explosive projectiles exploding on impact
- fixed player not being affected by knockback, which fixed some
entities not being able to attack
- added a controls menu that explains the controls when you click Play
- the Timer class now has two more parameters (boolean): autoUpdate and
autoReset
- separated potions into two categories: instant and duration (the
latter affects you for a specified length of time)
- moved all resources from the root folder to "resources"