Show the first, not the last, inserted item in wxListBox in wxOSX.

Show the first, not the last, inserted item in wxListBox in wxOSX.

The listbox showed its last, not first, item after creation in wxOSX which was
inconsistent with the other ports and generally inconvenient.

Fix this by ensuring that the first item being inserted is shown, and not the
last one as was (implicitly) the case before. A better fix would be to avoid
scrolling entirely but I don't know how to do this with NSClipView.

Closes #12365.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@66038 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775