Mplayers slave command osd_show_text assumes that text to display is encoded in utf8.
While writting command to slave process smplayer uses QString::toLocal8Bit() on this string
which will work on systems with utf8 locale. However it brakes when locale is non-utf8
and the text in OSD display is garbled. This patch fixes this situation making sure that
text is passed to slave process in utf8 encoding no matter what locale is used on users system.