Refs #8243 - Fix "browse to" default save directory.

Now, directories that are browsed to have a trailing separator, and
separators are forward-slashes rather than back-slashes.

A quick grep of "defaultsave.directory" seemed to reveal no cases where
adding a trailing separator would cause a problem.  Also, the Windows,
Linux and Mac command lines are all robust enough to parse/understand a
double forward slash anyway.

Lastly, changed IndirectAbsCor.py and IndirectBayes.py to use os.path.join
when working with the value of the defaultsave.directory property.  This
wasn't strictly necessary, but covers the case where a user manually types
in a default save directory whithout a trailing slash.
Merge remote-tracking branch 'origin/feature/8243_fix_browse_to_default_save_dir'