Added setting playback level

au4/src/au3wrap/CMakeLists.txt

au4/src/au3wrap/au3wrapmodule.cpp

au4/src/au3wrap/audacity3playback.cpp

au4/src/au3wrap/audacity3playback.h

au4/src/au3wrap/iaudacity3audiooutput.h

au4/src/au3wrap/iaudacity3playback.h

au4/src/au3wrap/internal/audacity3audiooutput.cpp

au4/src/au3wrap/internal/audacity3audiooutput.h

au4/src/playback/qml/Audacity/Playback/PlaybackToolBar.qml

...playback/qml/Audacity/Playback/internal/PlaybackLevel.qml

.../playback/qml/Audacity/Playback/internal/VolumeSlider.qml

au4/src/playback/view/toolbars/playbacktoolbarlevelitem.cpp

au4/src/playback/view/toolbars/playbacktoolbarlevelitem.h

au4/src/playback/view/toolbars/playbacktoolbarmodel.cpp

au4/src/playback/view/toolbars/playbacktoolbarmodel.h