Update: landmines, infinite boxes, minor improvements

What's new:

• New placeable: landmine. You can place it anywhere, but it will be affected by gravity (and it's quite bouncy as well, so take care - or not :). If a player or an NPC touches it it explodes and kills them instantly (optional time delay and blast radius might be added in the future).
• Added "roofs" for safe zones (also serves as protection from landmines placed directly above them).
• Blobs look better on angled platforms (they match their angles to the platform below them).
• Boxes will spawn from the top after all starting boxes has been picked up.

Fixed bugs reported by Krisz007 (big thanks!):

• In placement mode sawblades do not affect body parts anymore (it was possible to sweep them away).
• It was possible to place a sawblade under a safe zone in a way that it spawn-killed players; safe zones are now enlarged downwards slightly.