remove QtConcurrent dependency from JsCommandExecutor

Fewer dependencies makes bootstrapping easier.
Also, this prevents a potential deadlock on single core machines
when the executor is run through QtConcurrent, like in Qt Creator.

Task-number: QBS-68
Task-number: QBS-307

Change-Id: Id5383a2b5a6bf0d7880f0969e3730c3606e8b1d9
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>