power indicator: add other devices exposed by upower dbus interface

this uses https://github.com/nemomobile/nemo-qml-plugin-dbus and UPower dbus interface to list devices connected battery state

![](http://i.imgur.com/yOumuK0.png)
Let's use `device/battery_unknown`, `action/trend_up` (the closest thing to a power cord), and `device/battery_std` for the first three. No idea what UPS means.

See http://google.github.io/material-design-icons/