Get rid of wxMenu::m_startRadioGroup in wxOSX.

This code was probably copied from wxUniv but was wrong as we can't rely on
the items being always inserted in order. This commit on its own fixes
removing the first radio group menu item but it also makes possible to
properly implement the insertion of new items in the middle of an existing
radio group which couldn't be done with m_startRadioGroup approach at all.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74544 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/osx/menuitem.h

@@ -46,6 +46,9 @@ public:
     virtual void SetBitmap(const wxBitmap& bitmap) ;
     virtual const wxBitmap& GetBitmap() const { return m_bitmap; }
 
+
+    // Implementation only from now on.
+
     // update the os specific representation
     void UpdateItemBitmap() ;
     void UpdateItemText() ;
@@ -56,6 +59,19 @@ public:
     void SetRadioGroupStart(int start);
     void SetRadioGroupEnd(int end);
 
+    // return true if this is the starting item of a radio group
+    bool IsRadioGroupStart() const;
+
+    // get the start of the radio group this item belongs to: should not be
+    // called for the starting radio group item itself because it doesn't have
+    // this information
+    int GetRadioGroupStart() const;
+
+    // get the end of the radio group this item belongs to: should be only
+    // called for the starting radio group item, i.e. if IsRadioGroupStart() is
+    // true
+    int GetRadioGroupEnd() const;
+
     wxMenuItemImpl* GetPeer() { return m_peer; }
 private:
     void UncheckRadio() ;