Support drag from Finder to playlist on Mac.

Fix small build thing.

src/mac_startup.mm

@@ -34,7 +34,7 @@
 - (void) mediaKeyEvent: (int)key state: (BOOL)state repeat: (BOOL)repeat;
 @end
 
-@interface AppDelegate :NSObject <NSApplicationDelegate> {
+@interface AppDelegate :NSObject { //<NSApplicationDelegate> {
   QObject* application_handler_;
 }
 

src/playlist.cpp

@@ -338,7 +338,7 @@ QStringList Playlist::mimeTypes() const {
 }
 
 Qt::DropActions Playlist::supportedDropActions() const {
-  return Qt::MoveAction | Qt::CopyAction;
+  return Qt::MoveAction | Qt::CopyAction | Qt::LinkAction;
 }
 
 bool Playlist::dropMimeData(const QMimeData* data, Qt::DropAction action, int row, int, const QModelIndex&) {