Wireless Wires Bugfix

- This removes the dreaded wireless wires bug, which caused wires disconnected by explosions to actually still conduct power until powernet was manually updated.
- This is done by actually calling makepowernets() after explosion is processed. Since makepowernets is quite laggy on it's own, and defer_powernet_rebuild is suposed to limit lag, i have added check for explosion strength. Weak explosions will no longer use defer powernet rebuild, because amount of affected wires will likely be very low, and makepowernets() would actually lag more than just del() on few cables.
- This also adresses issue with (on bay currenly unused) singularity, which exhibits similar behavior when eating wires (wireless wires bug)