fixing reading from/to XML file

WARNING current subversion tree hasn't a functional way of saving files or reading from it. Whereby most bugfixes were probably done you may experience incompatibelities while checking out from subversion.

The new DTD is more flexible and robust than the previous one. It was created in toughts of future releases. But definitive 0.4.0 release is still not released.

Regarding the application level of the Advanced Gtk+ Sequencer code may need more work to be done to fit the needs of the new XML API.