MussaSetupDialog remembers previous load directory

Implements ticket:101
Basically the setup dialog kept using the current directory, this
patch stores a "default_directory" in the SequenceSetupFrame so the
various SequenceSetupWidgets have a place to grab a default directory.

It starts off set to the "current" path, but whill remember whatever
you pick after that. (Perhaps it should default to the home directory?)