FR: New Thread API doesn't have a way to set the priority of lambda threads

This used to be a completely valid thing to do. It’s not possible with the new API. Instead Thread::launch should take an optional second parameter that is the priority.

    juce::Thread::launch ( [ &synth, exp, options, progressCallback ]
    {
       juce::Thread::setCurrentThreadPriority ( 3 );
       DBG("Yowza!");
    }