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

modules/gui/qt4/components/preferences_widgets.cpp

@@ -341,7 +341,7 @@ FileConfigControl::FileConfigControl( vlc_object_t *_p_this,
 
 void FileConfigControl::updateField()
 {
-    QString file = QFileDialog::getOpenFileName( NULL,
+    QString file = QFileDialog::getSaveFileName( NULL,
                   qtr( "Select File" ), QVLCUserDir( VLC_HOME_DIR ) );
     if( file.isNull() ) return;
     text->setText( toNativeSeparators( file ) );