Qt: allow to select inexistent files from Prefs
Qt: allow to select inexistent files from Prefs

 - for some reason, this worked on Linux (?!?)
 - for some other reason, there doesn't seem to be 2 different modes for add_file

Close #4004

vlc-mirror/vlc,vlc-mirror/vlc,vlc-mirror/vlc,krichter722/vlc,xkfz007/vlc,krichter722/vlc,shyamalschandra/vlc,vlc-mirror/vlc,jomanmuk/vlc-2.2,jomanmuk/vlc-2.1,vlc-mirror/vlc-2.1,krichter722/vlc,vlc-mirror/vlc-2.1,krichter722/vlc,jomanmuk/vlc-2.2,shyamalschandra/vlc,xkfz007/vlc,krichter722/vlc,vlc-mirror/vlc,krichter722/vlc,jomanmuk/vlc-2.1,jomanmuk/vlc-2.1,jomanmuk/vlc-2.2,jomanmuk/vlc-2.2,xkfz007/vlc,jomanmuk/vlc-2.2,vlc-mirror/vlc,jomanmuk/vlc-2.1,xkfz007/vlc,jomanmuk/vlc-2.1,vlc-mirror/vlc-2.1,vlc-mirror/vlc-2.1,xkfz007/vlc,xkfz007/vlc,krichter722/vlc,shyamalschandra/vlc,jomanmuk/vlc-2.2,shyamalschandra/vlc,vlc-mirror/vlc,vlc-mirror/vlc-2.1,shyamalschandra/vlc,vlc-mirror/vlc-2.1,shyamalschandra/vlc,jomanmuk/vlc-2.2,xkfz007/vlc,vlc-mirror/vlc-2.1,jomanmuk/vlc-2.1,jomanmuk/vlc-2.1,shyamalschandra/vlc