Draft: improve the audio vizualiser

TODO:

• Add time axis
• Add a way to zoom in / zoom out on X axis
• Add a way to change "audio sensibility" for spectrum
• Add the display of timed lines/syl
• Make the timing Bar draggable and reflect the changes in the ASS
• Maybe add lots of rainbowish colors to the spectrum because rainbows are great

src/Lib/Document/VivyDocument.cc

@@ -135,6 +135,14 @@ VivyDocument::getAssSubDocument() const noexcept
     return assDocument;
 }
 
+AudioContext::StreamPtr
+VivyDocument::getAudioStream() const noexcept
+{
+    if (auto p = audioDocument.get())
+        return audioDocument->getDefaultStream();
+    return AudioContext::StreamPtr();
+}
+
 bool
 VivyDocument::checkDocumentCapabilities(Capabilities asType) const noexcept
 {