better handling of error conditions in all BuildFromXML methods, per comment:
// BuildFromXML methods should always return a BlockFile, not NULL,
// even if the result is flawed (e.g., refers to nonexistent file),
// as testing will be done in DirManager::ProjectFSCK().
Also some minor refactoring.
This feature works with mp3 right now - may work on other formats, but that will be a seperate commit.
It is not enabled so most of the changes won't even be compiled, and those that do won't be run.
To enable it uncommment the EXPERIMENTAL_ODFFMPEG def in Experimental.h
