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)
i'm not sure we need all the icons, for example for the power cord we just need to set the indicator icon, there is no need for a line in the dropdown. as for the battery i haven't looked into it because i currently don't have one :/