droppable silent packets are added for dropped packets;

_currentJitterBufferFrames replaced with moving average of frames
available.  Downstream framesAvailable stat now includes the frames in
QAudioOutput as well as the ringbuffer.
changed format label string a bit
renamed a function
re-added calculated desired frames to domain page stats
removed space
case statement style fix
Merge pull request #3179 from wangyix/quick_audio_PR

Lost audio packets are replaced with silent frames in AudioMixer; AudioMixer silent-packet-dropping now enabled no matter what; Moving avg of audio frames available replaces _currentJitterBufferFrames