Use dictionary sort for wxListBox strings.

This should result in more expected, and more consistent with MSW, order of
the items differing in case only.

Also do the same for wxChoice in wxGTK1.

Closes #16356.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76902 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775