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/db/plugins/simple/SongSort.cxx

@@ -96,7 +96,7 @@ song_cmp(const Song &a, const Song &b) noexcept
 		return ret < 0;
 
 	/* still no difference?  compare file name */
-	return IcuCollate(a.uri.c_str(), b.uri.c_str()) < 0;
+	return IcuCollate(a.filename.c_str(), b.filename.c_str()) < 0;
 }
 
 void