Updates emergency and wall shield power usage
Updates emitters and containment field power usage

As well as PA power usage. It's not used at the moment, but it might be
used by downstream or in the future.
Updates renwick shield generator
Fixes #5935

Also improves apc/process() (doesnt add and then remove charge from
cells when it doesn't have to).
Fixes icons and other things not updating when the master controller updates power status.

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 #5901
Reorganizes shieldgen source code and icon files
Merge pull request #5940 from mwerezak/power-net

Update for shield generator power use and power net fixes