Remove the ExecutionContext from some signatures, replace it with the engine in others

Remove the ExecutionContext from some signatures, replace it with the engine in others

Most places we actually only need the engine, not the context. Fix this
in our internal APIs. Also remove the context where it's not required.

Change-Id: Ib6281e7b9de9124612a9df24df9bedf47559b159
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>