Tasking::parallel should default to "QThread::idealThreadCount()", not infinite

Tasking::parallel currently equals Tasking::parallelLimit(0) which does not limit the amount of spawned tasks at all. This is rarely what is wanted as it can easily clog the system to a point where the UI starts to be blocked, or system resources are used up.

I'd suggest to change parallel to mean "QThread::idealThreadCount()", and maybe add "inifiniteParallel" for "as many tasks as you like".