LibAudio: Add a generic audio metadata container

This container has several design goals:
- Represent all common and relevant metadata fields of audio files in a
  unified way.
- Allow perfect recreation of any metadata format from the in-memory
  structure. This requires that we allow non-detected fields to reside
  in an "untyped" miscellaneous collection.

Like with pictures, plugins are free to store their metadata into the
m_metadata field whenever they read it. It is recommended that this
happens on loader creation; however failing to read metadata should not
cause an error in the plugin.

Userland/Libraries/LibAudio/CMakeLists.txt

@@ -4,6 +4,7 @@ set(SOURCES
     WavLoader.cpp
     FlacLoader.cpp
     WavWriter.cpp
+    Metadata.cpp
     MP3Loader.cpp
     QOALoader.cpp
     QOATypes.cpp