[Azoth OTRoid] Use OTRL_PRIVKEY_FPRINT_HUMAN_LEN instead of hardcoded value

if it is available. Details:
https://sourceforge.net/p/otr/libotr/ci/e31c21d/tree/src/privkey.h?diff=fe57dd8
[Azoth OTRoid] Call GetOTRFilename() outside of QtConcurrent::run().
Merge pull request #317 from tehnick/otroid

[Azoth OTRoid] Small improvements.