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

@@ -13,7 +13,7 @@ class AudioVisualizer final : public QWidget {
     Q_OBJECT
 
 public:
-    AudioVisualizer(QWidget *parent = nullptr) noexcept;
+    explicit AudioVisualizer(QWidget *parent = nullptr) noexcept;
     ~AudioVisualizer() noexcept = default;
 
     static AudioVisualizer *fromFile(const QString &);