Separate "turn off display when inactive for..." values for plugged in vs. on battery

Currently, there is only one setting for how long the display stays on when the computer is inactive, and this applies regardless of whether the computer is plugged in or is on battery power.

It would be very helpful to be able to set two different values for this: one for when the computer is plugged in (and power consumption isn't a concern) and one for when it's on battery (when I want the battery to last as long as possible).