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)
On my computer, the power cord has no vendor or model, and the progress is always 0. So it's pretty useless. What we really want instead is to know what the primary battery is and whether it's being charged (for use by the indicator). Then we show the primary batter and any other device batteries in the popover.