Blocking calls when setting brightness causing input events to queue.

The call to powerd which changes the brightness is synchronous, which is blocking the input events. This results in input events to wait in the queue until the call is completed.

Test case:
Brightness slider in the indicator lags behind while swiping around a lot.

Result: (from MIR_CLIENT_INPUT_RECEIVER_REPORT=log)
http://pastebin.ubuntu.com/10677265/

Trial Resolution:
=== modified file 'src/powerd_mediator.cpp'
--- src/powerd_mediator.cpp 2014-09-17 09:44:55 +0000
+++ src/powerd_mediator.cpp 2015-03-25 14:20:00 +0000
@@ -195,7 +195,7 @@
     normal_brightness = brightness;

     if (backlight_state != BacklightState::automatic)
- powerd_interface->call("setUserBrightness", brightness);
+ powerd_interface->asyncCall("setUserBrightness", brightness);
 }

I'm not sure about implications of changing to async calls though.