decoder_plugin: scan tags with callback table

Pass a callback table to scan_file() and scan_stream(), instead of
returning a tag object.

src/tag_ape.h

@@ -20,6 +20,19 @@
 #ifndef MPD_TAG_APE_H
 #define MPD_TAG_APE_H
 
+#include <stdbool.h>
+
+struct tag_handler;
+
+/**
+ * Scan the APE tags of a file.
+ *
+ * @param path_fs the path of the file in filesystem encoding
+ */
+bool
+tag_ape_scan2(const char *path_fs,
+	      const struct tag_handler *handler, void *handler_ctx);
+
 /**
  * Loads the APE tag from a file.
  *