Fix insertion of radio menu items in wxOSX wxMenu.

Fix insertion of radio menu items in wxOSX wxMenu.

Deal correctly with updating the indices when a radio item is inserted into an
existing radio group (which wasn't done previously and resulted in a unit test
failure in MenuTestCase::RadioItems()) and also with inserting the normal
items before an existing radio group as the stored indices were not updated
correctly.

The code is still ugly and it probably wouldn't be a bad idea to reuse
wxMenuRadioItemsData used in wxMSW for similar purposes, but at least the unit
tests pass now.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74548 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775