* cleaned, organized and commented a large part of the power code,
* new procs handling : powernets creation/destruction, cable adding/removing, machines adding/removing,
* new cable laying possibility : if you click in the turf you're standing on, you'll try to lay a node cable in the direction you're facing,
* cleaned oversights that could (and would) leaved an empty powernet in the powernets global list
* rewrote the cut_cable and propagate_network procs to correctly updates every connected cables and machines (fix, at last, #1455)
* others fixes and polish...
Merge pull request #3633 from Menshin/powernets_refactoring

[RFC] Powernets refactoring