Add support for non-binary LTspice XVII files

[PeterFeicht]

When adding support for UTF-16 encoded LTspice XVII files I completely forgot about non-binary files, which would encode the data as UTF-16 as well. This is not supported right now and should be added.

Note: MATLAB's fopen function doesn't support UTF-16 officially, though it has been reported to work nonetheless.

[PeterFeicht]

Text .raw files don't seem to use UTF-16 after all.