db/simple/Song: rename "uri" to "filename"

This attribute is not a URI; it is just the filename without its
parent directory path.  To avoid confusion, let's rename it to
"filename", leaving the struct without a "uri" attribute.

src/SongUpdate.cxx

@@ -110,7 +110,7 @@ Song::UpdateFileInArchive(ArchiveFile &archive) noexcept
 {
 	assert(parent.device == DEVICE_INARCHIVE);
 
-	std::string path_utf8(uri);
+	std::string path_utf8(filename);
 
 	for (const Directory *directory = &parent;
 	     directory->parent != nullptr &&