[Feature] Stackable Inventory Items

 - [x] Should only take up 1 inventory slot
 - [x] Should show stack count in inventory
 - [x] Should be able to split the stack
 - [x] Should be able to add to item bar
 - [x] Should show stack size in item bar
 - [x] Should be able to pick up half the stack
 - [x] Should be able to pick up one item off the stack
 - [x] Should be able to pick the whole stack