Revision 3478

* Fix: Don't automatically delete RIFF chunks from DLS/gig classes'
  destructors. Added new virtual method DeleteChunks() to those classes
  for this which must be explicitly called instead to remove their RIFF
  chunks.
* Fix: Many methods of DLS/gig classes assumed a RIFF chunk read position
  of zero; which is unsafe per se.
* Added C++11 "override" keyword where appropriate.
* DLS.cpp, DLS.h: Added new abstract interface base class DLS::Storage
  which is derived by the respective classes for implementing (the old)
  UpdateChunks() and the new DeleteChunks() method.
* RIFF.cpp, RIFF.h: Added new method progress_t::subdivide().
* Bumped version (4.1.0.svn13).