Refactor tagging and add library scanning

- Replace `Tag` class with utility helpers for hashing and file type parsing
- Change `Track.artwork` from `std::string` to `TagLib::VariantMap`
- Add `importArtwork`, rename `importFolder` to `scanFolder`
- Add `scanLibrary`, `updateLibrary`, `editMetadata`, and `seek` methods to `Player` hierarchy
- Remove obsolete `Tag.h`/`Tag.cpp` files

include/Player.h

@@ -17,6 +17,9 @@ class Player {
 
     virtual void initPlayer ();
     virtual void loadFile ( Track track );
+    virtual void scanLibrary();
+    virtual void updateLibrary();
+    virtual void editMetadata();
 
     // Transport Control
     virtual void play ();
@@ -24,6 +27,7 @@ class Player {
     virtual void pause ();
     virtual void previous ();
     virtual void next ();
+    virtual void seek();
 
     virtual ma_uint64 getPlayheadPosition ();
     virtual void setPlayheadPosition () const;
@@ -44,6 +48,7 @@ class Player {
   protected:
     PlaybackEngine PE_;
     trackVec queue_;
+    std::vector<Track> library;
 
   private:
     std::string artist_;