Player inventory (initial version)

[patowen]

Partial fix for #397

This PR adds the initial version of an inventory component for players to store blocks they break. The following features exist:

• An ability to turn itself on and off, controlled by the gameplay_enabled sim_config property
• The GUI that tells you the quantity of the selected material you have. Since not all materials are selectable yet, if you break an unselectable material, it effectively disappears into the void.
• A server-authoritative inventory that gets synchronized with the client

The following features are left to future work:

• The ability to select any material you have on you
• A GUI that shows you everything you're holding at once
• Serialization of the inventory for saving
• Prediction of inventory contents (Required for the player to be able to place more than one block of the same material before receiving server confirmation. This feature likely should be added together with prediction of the world's block updates.)