Qt4: ask for confirmation before overwriting files with sout

Both conversion and streaming output wizard are covered.
However, demuxdump is _not_ covered.

modules/gui/qt4/components/sout/sout_widgets.cpp

@@ -128,6 +128,7 @@ QString FileDestBox::getMRL( const QString& mux )
         }
     }
     m.option( "dst", outputfile );
+    m.option( "no-overwrite" );
     m.end();
 
     return m.getMrl();

modules/gui/qt4/dialogs/convert.cpp

@@ -140,7 +140,7 @@ void ConvertDialog::close()
         mrl += ":";
         if( displayBox->isChecked() )
             mrl += "duplicate{dst=display,dst=";
-        mrl += "std{access=file,mux=" + profile->getMux()
+        mrl += "std{access=file{no-overwrite},mux=" + profile->getMux()
              + ",dst='" + fileLine->text() + "'}";
         if( displayBox->isChecked() )
             mrl += "}";