Merge lp://staging/~unity-api-team/indicator-power/lp-1370791-15.05-adjust-slider-when-brightness-is-changed-by-powerd into lp://staging/indicator-power/15.04

When in autobrightness mode, keep the brightness slider's value in sync with the system brightness

== Description of the Change

In the brightness code, add a powerd proxy that listens to property changes from the com.canonical.powerd.Brightness property. When this changes, update the indicator's power slider to reflect the change.

== Checklist

* Are there any related MPs required for this MP to build/function as expected? Please list.

Yes, https://code.launchpad.net/~charlesk/powerd/lp-1370791-add-brightness-property is required to publish powerd's brightness changes on the bus.

> * Is your branch in sync with latest trunk? (e.g. bzr pull lp:trunk -> no changes)

Targeted for lp:indicator-power/15.04. bzr pull lp:indicator-power/15.04 -> no changes

> * Did the code build without warnings?

Yes

> * Did the tests run successfully?

Yes

> * Did you perform an exploratory manual test run of your code change and any related functionality?

Yes

> * If you changed the packaging (debian), did you subscribe the ubuntu-unity team to this MP?

N/A

> * Did your component test plan pass? If on a device, what image number?

Yes mako wily r202

> * Please list which manual tests are germane for the reviewer in this MR.

indicator-power/device-brightness-slider-auto and
indicator-power/device-brightness-slider

> Did you provide a link to this page https://wiki.ubuntu.com/Process/Merges/Checklists/indicator-power

Yes