Abstract inventory code to allow defining a player inventory on the top instead of the bottom

[valkyrienyanko]

The code should be made to easily switch between the 2 depending on what kind of game you want to make.

If the player inventory is defined to be at the top then...

• the chest inventory is always defined at the bottom
• the hotbar should be defined to be the very first row instead of the very last row
• the inventory exit animations should initially move upward instead of downward (and so on for the re-entry animation)