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/Lib/Document/VivyDocument.cc

@@ -231,7 +231,7 @@ VivyDocument::detectDocumentType(const QFileInfo &file, Capabilities *ableType)
 }
 
 void
-VivyDocument::rename(const QString &newName)
+VivyDocument::copy(const QString &newName)
 {
     const QString newNameWithExtension =
         (newName.right(fileSuffix.size()) == fileSuffix) ? newName : newName + "." + fileSuffix;
@@ -250,7 +250,7 @@ VivyDocument::rename(const QString &newName)
     // Compute new paths, the document is really on disk initially
     else {
         qDebug() << "Renaming a real file";
-        renameWith(newPath, [=, this]() noexcept -> void {
+        copyWith(newPath, [=, this]() noexcept -> void {
             documentLocation = newPath.dir();
             documentName     = newPath.baseName();
         });