SongFilter: convert to a C++ class

src/DatabaseQueue.hxx

@@ -23,12 +23,12 @@
 #include "gcc.h"
 #include "gerror.h"
 
-struct locate_item_list;
+class SongFilter;
 struct player_control;
 
 gcc_nonnull(1,2)
 bool
 findAddIn(struct player_control *pc, const char *name,
-	  const struct locate_item_list *criteria, GError **error_r);
+	  const SongFilter *filter, GError **error_r);
 
 #endif