Stop syncthing when laptop power changes to battery mode

[rutkas]

Syncthing can drain battery fast (not only transfers, but folder scannings eat much CPU). So it would be great to have an option that stops syncthing when computer goes on battery mode and starts it when it goes back on external power (of course in the case if it was stopped manually, we do not want to start it).

Same functionality is present in MS OneDrive and used by default.
This is no related to metered network or to turning off folders/clients, just stop/start syncthing on power change events to save battery life.

As for me, I stop synctrayzor manually each time when switch off external power on laptop, but sometimes forget to turn it back on.