Open non-ASCII filenames via the command line

I fixed so that files containing file non-ASCII characters can be opened
via the command line by converting the std::string to a QString by using
QString::fromUtf8(string.c_str()) instread of QString::fromStdString(string).

Non-ASCII characters are not a problem outside the command line, but
I do not known how it is on the command line options.

This is tested on GNU/Linux, I do not know have it will work on other
systems, but it should work since QString::fromStdString(string) did an
incorrect job of creating QString (because it is encoding unaware.)

Signed-off-by: Mattias Andrée <maandree@operamail.com>