implement stereo vu meters and bugfixes for faux-stereo pan

* vu meters and channel names now update asynchronously via storage tree values
* master VU is only approximate
* added vu params to channels store and VU_LEFT_ID and VU_RIGHT_ID constants
* added LinJamConfig::makeStereoName() method
* added LinJam::UpdateLoopProgress() , UpdateVuMeters() , UpdateRecordingTime() methods