Remove filename extension for chapter mark file

The tool mp4chaps by the following command

$ mp4chaps -i test.mp4

looks for the file "test.chapters.txt" to read the chapter marks
from. So removing the original file name extension (e.g. wav or flac)
makes it slightly more convenient for the user.

libs/ardour/export_handler.cc

@@ -520,7 +520,10 @@ ExportHandler::get_cd_marker_filename(std::string filename, CDMarkerFormat forma
 	case CDMarkerCUE:
 		return filename + ".cue";
 	case MP4Chaps:
-		return filename + ".chapters.txt";
+	{
+		unsigned lastdot = filename.find_last_of('.');
+		return filename.substr(0,lastdot) + ".chapters.txt";
+	}
 	default:
 		return filename + ".marker"; // Should not be reached when actually creating a file
 	}