made Audio::_ringbuffer 10 frames for nonWindows;

fixed/separated framesAvailable stats for _ringBuffer and _audioOutput;
made QAudioOutput overflow check disabled by default
separated ringbuffer and outputbuffer avg framesAvailable stats in interface
changed menu item name for QAudioOutput overflow check
Merge pull request #3186 from wangyix/master

made Audio::_ringBuffer size 10 frames for non-Windows builds; improved interface downstream audio framesAvailable stat; disable QAudioOutput overflow check by default