hard coding of UTF-8 as export format for plain text

[TeresaM12]

Problem: The native encoding of treeline seems to be utf-8. If the OS encoding is set differently (for what ever reason this is common in Windows) plain text export is not possible as there exists no transcoding function instead the export will fail.

So as long as no transcoding is done, hard coding the plain text export to utf-8 seems like a sensible solution, which is also used for treeline, xml, html exports.

[doug-101]

I decided to address this a bit differently. TreeLine still attempts to export or import using the OS system locale settings. If it sees a Unicode error, it silently falls back to utf-8 encoding. I think this gives a good compromise, since it uses the OS default when it's sensible, but avoids an error when it isn't.

Thanks,
Doug