Qt: don't allow negative screen-numbers
Qt: don't allow negative screen-numbers

Close #13643

vlc-mirror/vlc,xkfz007/vlc,shyamalschandra/vlc,shyamalschandra/vlc,xkfz007/vlc,xkfz007/vlc,krichter722/vlc,vlc-mirror/vlc,krichter722/vlc,shyamalschandra/vlc,vlc-mirror/vlc,xkfz007/vlc,vlc-mirror/vlc,krichter722/vlc,shyamalschandra/vlc,xkfz007/vlc,krichter722/vlc,krichter722/vlc,krichter722/vlc,xkfz007/vlc,vlc-mirror/vlc,krichter722/vlc,vlc-mirror/vlc,shyamalschandra/vlc,xkfz007/vlc,shyamalschandra/vlc,vlc-mirror/vlc,shyamalschandra/vlc