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/VivyApplication.cc

@@ -5,6 +5,7 @@
 #include <QIcon>
 #include <QFontDatabase>
 #include <locale>
+#include <QTextCodec>
 
 using namespace Vivy;
 
@@ -35,7 +36,8 @@ VivyApplication::setTheme(Theme theme) noexcept
 int
 VivyApplication::exec() noexcept
 {
-    // For MPV
+    // For MPV & Qt
+    QTextCodec::setCodecForLocale(QTextCodec::codecForName("UTF-8"));
     std::setlocale(LC_NUMERIC, "C");
 
     // Add fonts