Add Low power mode toggle in D-Bus API

A "low power mode" boolean property would be very useful in upower. It could be used to tell applications and parts of the system that they should try to conserve power.

We could make this would automatically kick in when the battery is below a certain level, or we could have the user session in control of it (eg. gsd-power). The end-user should also be able to force that toggle on, as they would know better than the computer when the next they get to recharge their system is, and also turn it off, if they want to use their system normally until the battery's depleted.

CC @pwithnall