Adjust Exec clause in desktop file to be able to properly open .sgf files

Looks like with no % code %f is implied (eg. thunar launches one qgo
process per selected .sgf file).  With an explicit %F it does create a
single qgo process for several files.
Merge pull request #32 from ydirson/master

Adjust Exec clause in desktop file to be able to properly open .sgf files