Add the preferLowPowerToHighPerformance context attribute
Move preferLowPowerToHighPerformance into 1.0 latest spec
as suggested by Ken. Copy declaration into 2.0.
preferLowPowerToHighPerformance in 2.0 IDL also.
Merge pull request #378 from grorg/preferLowPowerToHighPerformance

Add the preferLowPowerToHighPerformance context attribute