Correct qTox.desktop

With an additional ';' installing qTox desktop file resulted in warning:

/usr/share/applications/qTox.desktop: error: value "InstantMessaging;;AudioVideo;Network;" for key "Categories" in group "Desktop Entry" contains an unregistered value ""; values extending the format should start with "X-"
Italian translation: update
Merge branch 'pr667'
Merge branch 'pr669'