power: Battery charging thresholds

Some laptop models offer the ability to set charging thresholds, to held extend the lifespan of a battery.

It was questioned in a UPower issue what (if any) interface Gnome should provide for this. It was also noted there that this feature was recently added to KDE.

At System76, we intend to support this in Pop!_OS, whether or not it's provided upstream, since we have recently added support for charging thresholds to our open firmware.