BUG: Set scene to MRB directory instead of temp

When loading an MRB the MRML scene url was left pointing to
the deleted temporary directory. This change resets the URL
to point to a mrml file in the same location as the MRB so
that on subsequent saves the default location is correct.
Since the temporary directory is deleted after loading,
mark all the storable nodes as modified since read so that
they will be properly flagged in the save data dialog for
writing.
Support relative paths for loading MRBs (the unzip call
needs an absolute path).

Issue 0003698

From: Nicole Aucoin <nicole@bwh.harvard.edu>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24046 3bd1e089-480b-0410-8dfb-8563597acbee