wxOSX: store the menu title when inserting it in the menu bar.

We "lost" the menu title if the menu was inserted, and not appended, to the
menu bar, resulting wxMenuBar::GetMenuLabel[Text]() returning empty strings
for it in this case.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73870 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775