MISC: Move UI related files into their own folder

CMakeLists.txt

src/Main.cc

src/UI/AudioVisualizer.cc

src/UI/AudioVisualizer.hh

src/UI/DialogHelp.cc

src/UI/DialogHelp.hh

src/UI/MainWindow.cc

src/UI/MainWindow.hh

src/UI/TimingBar.cc

src/UI/TimingBar.hh

src/UI/TimingScene.cc

src/UI/TimingScene.hh

src/UI/TimingView.cc

src/UI/TimingView.hh

src/UI/VivyDocumentView.cc

src/UI/VivyDocumentView.hh

src/VivyApplication.cc