Update for shield generator power use and power net fixes

* Shield generators now use power
* Fixed up renwick shield generators, they could probably be moved out of the WorkInProgress folder now.
* Fixed issue with power updates not updating icons among other things. This was due to the master controller using update_powered_status() instead of power_change(), but many machines override power_change() to do various things when the machine gains or loses power.
* Fixes power machinery not determining how much power is actually available in a power net, or determining it incorrectly. Powernets now have proper procs for drawing power.
* Improves apc process code. Somewhat more sane and doesn't continually add and remove charge from power cells when it doesn't need to.