Resolve invalid base path as part of WB-765 Upload Scene File to BALSA Dialog...

Resolve invalid base path as part of WB-765 Upload Scene File to BALSA Dialog Updates: Code that computed automatic base path just found the longest "string" prefix in the paths to all data files.  If  two files were in sibling directories whose directory names started with the same characters (/user/files/bill  /usr/files/bob) the path was "/usr/files/b" (longest string match but not a directory).  So, use the longest string match, but it if is an invalid directory, use the parent directory as the base path and in this example is "/usr/files".  Also renamed some methods and a member to indicate that they are only used for the custom base bath.