Fix inserting radio menu items in wxGTK too.

Fix inserting radio menu items in wxGTK too.

After fixing the insertion of radio menu items in wxMSW, also do it for wxGTK
to make the newly added unit test pass there as well.

Remove the unneeded wxMenu::m_prevRadio which doesn't make any sense neither
(just as the "current radio group" pointer removed from wxMSW code before) and
simply use the radio group of the existing item this radio item is being
inserted before or after instead.

See #13200.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@67721 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775