Implement the VivyDocument specification

Load and write to VivyDocuments.

While at it, refactor of {Audio,Video}Context and their streams and fix all warnings in headers with clang. Only place external headers in the precompiled header, not Vivy's ones.

Includes a small API update for the VideoSubDocument (ass the VideoContext with some getters).

Depends on !19 (merged)

src/UI/AbstractDocumentView.hh

@@ -43,6 +43,7 @@ public:
 
 signals:
     void viewActionsChanged();
+    void documentPropertyChanged();
 
 protected:
     void deleteAllContent() noexcept;