Add documents

Add documents, document views and a document store to manage document and which one is displayed.

• The MainWindow make calls to the VivyDocumentStore to create the documents and the views
• The VivyDocumentStore create the documents
• The VivyDocumentView is only here to have a view, it won't hold any data
• The VivyDocument can be manipulated in any part of the code
• Widgets view the VivyDocument through the VivyDocumentView
• Add the VivyApplication that will hold the window and the non-graphic data
• The main window can get the currently selected main document

src/AudioVisualizer.h

 #ifndef VIVY_AUDIOVISUALIZER_H
 #define VIVY_AUDIOVISUALIZER_H
 
+#ifndef __cplusplus
+#error "This is a C++ header"
+#endif
+
 #include "TimingView.hpp"
 #include <QWidget>