Update: placeable walls, NPC enemies, minor enhancements

What's new:

• Added 2 new placeable objects: small and large platform/wall. They can also be rotated in placement mode with gamepad trigger buttons, Q/E ("place" has been moved to "F" because of that) or numpad 7/9.
• The first NPC enemy is implemented: the bug, which slowly starts walking and changes direction when it walks into a wall. It also wraps, so bugs falling through gaps in the ground will re-enter from the top. A new bug spawns every 47 seconds (don't ask why), it randomly falls from above; freshly spawned bugs don't kill players while in mid-air, only after they landed.
• Slightly modified head jump mechanics: smaller "soft spot" (which is now hidden), automatic jump after hit (which also resets double-jump)
• Boxes falling through gaps in the bottom will re-enter from the top (as well as players)
• Bugfix: head stomps and TNT kills didn't count as kills, they both do now.