Fix item index in wxRadioBox events in wxQt

Use 0-based index for the radio box items, as in the other ports,
instead of auto-generated values.

Closes https://github.com/wxWidgets/wxWidgets/pull/1127

src/qt/radiobox.cpp

@@ -116,10 +116,12 @@ static void AddChoices( QButtonGroup *qtButtonGroup, QBoxLayout *qtBoxLayout, in
     Button *btn;
     bool isFirst = true;
 
+    int id = 0;
+
     while ( count-- > 0 )
     {
         btn = new Button( wxQtConvertString( *choices++ ));
-        qtButtonGroup->addButton( btn );
+        qtButtonGroup->addButton( btn, id++ );
         qtBoxLayout->addWidget( btn );
 
         if ( isFirst )