Add charge limit to supported battery models

[pastaq]

Some batteries provide a sysfs character file to limit charging to a specified percentage and only start again after reaching another. We should expose sliders in PowerTools to set the charge limit if /sys/class/power_supply/BAT*/charge_control_end_threshold and/or /sys/class/power_supply/BAT*/charge_control_start_threshold exist to allow users to limit battery degradation while docked.

[pastaq]

This issue is more complicated and will need some different logic for different devices.