power-profiles-daemon has the ability to detect battery change events
using upower and to apply different tuned settings based upon whether
plugged into power or not.
In PPD this is used specifically to set the energy performance preference
differently in the 'balanced' profile, but there is no reason that this
concept can't actually apply to all profiles.
Add support for detecting battery change events and apply a profile
specified in ppd.conf for battery in the different PPD states.