replaced recently added wxMBConv::GetMBNul() with a less clever but better

replaced recently added wxMBConv::GetMBNul() with a less clever but better
working GetMinMBCharWidth(): the idea is that we can't deal with completely
arbitrary encodings anyhow using the current API as we get confused by NUL
runs in the middle of the string, so instead just deal correctly with normal
multibyte encodings, UTF-16/UCS-2 and UTF-32/UCS-4 which should cover 99.9% of
the cases

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@38523 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775