power: Use icons from UPower

https://bugzilla.gnome.org/show_bug.cgi?id=710393
power: Fix build

up_client_enumerate_devices_sync() was removed.

https://bugzilla.gnome.org/show_bug.cgi?id=710393
power: Fix run-time for DeviceChanged signal removal

The DeviceChanged signal was removed, instead, each interesting
device should be monitored individually. This means we need to keep
our own device list locally.

https://bugzilla.gnome.org/show_bug.cgi?id=710393