add operator+=(unsigned char) for backwards compatibility and because we generally...

add operator+=(unsigned char) for backwards compatibility and because we generally handle both signed and unisnged chars everywhere; added wxString(const wxCStrData&, size_t) ctor to resolve ambiguity between wxString(const char *, size_t) and wxString(const unsigned char *, size_t) and wxString(const wxString&, size_t) to resolved embiguity for string parameter of ctor

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@45017 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775