Update: new pickup system, TNT, gaps

What's new:

• Basic inventory system: players don't have to place picked up items immediately anymore. Instead, when they pick up a box, the item will be held in their hand and they can press the "place" button anytime to go into "place mode". Players can only carry one item at a time and they won't pick up a new item if they already have an item in their hands. If a player dies while holding an item, the item will be dropped and can be picked up again by any player.
• New "placeable" object: TNT. It explodes after a few seconds destroying other placed objects and players in its radius.
• Vertical wrap: there are gaps in the ground, players falling through them will re-enter the level from the top.
• Boxes are slightly bigger.
• Safe zones are slightly smaller.