issue 84: implement conversation log export

[pospeselr]

@m-simonelli changes from your PR:

• fixed bug in to_string(tego_file_hash_t* const)
• fixed file transfer UI issues with added 'Accepted' state
• logging timestamps are all calculated by the local user (ie timestamps from messages are ignored)
• tweaked Export Conversation UX: log file is offered to be saved as ~/Downloads/$contact-$timestamp.log tweaks to log output format
• some internals cleanup, timestamps are saved as QDateTime on the base Event struct
• cancelling log save does not trigger error box
• fixed file transfer UI issues with added 'Accepted' state