DetachedSong: fork of struct Song

From now on, struct Song will be used by the database only, and
DetachedSong will be used by everybody else.  DetachedSong is easier
to use, but Song has lower overhead.

src/SongFilter.cxx

@@ -20,6 +20,7 @@
 #include "config.h"
 #include "SongFilter.hxx"
 #include "Song.hxx"
+#include "DetachedSong.hxx"
 #include "tag/Tag.hxx"
 #include "util/ASCII.hxx"
 #include "util/UriUtil.hxx"
@@ -151,6 +152,18 @@ SongFilter::Item::Match(const Song &song) const
 	return song.tag != NULL && Match(*song.tag);
 }
 
+bool
+SongFilter::Item::Match(const DetachedSong &song) const
+{
+	if (tag == LOCATE_TAG_BASE_TYPE)
+		return uri_is_child_or_same(value.c_str(), song.GetURI());
+
+	if (tag == LOCATE_TAG_FILE_TYPE)
+		return StringMatch(song.GetURI());
+
+	return Match(song.GetTag());
+}
+
 SongFilter::SongFilter(unsigned tag, const char *value, bool fold_case)
 {
 	items.push_back(Item(tag, value, fold_case));
@@ -203,6 +216,16 @@ SongFilter::Match(const Song &song) const
 	return true;
 }
 
+bool
+SongFilter::Match(const DetachedSong &song) const
+{
+	for (const auto &i : items)
+		if (!i.Match(song))
+			return false;
+
+	return true;
+}
+
 std::string
 SongFilter::GetBase() const
 {