Preparing wxString for UTF-8 representation:

Preparing wxString for UTF-8 representation:
1) split into wxStringImpl class that has std::string-like API and operates on char* or wchar_t* data and wxString class that provides Unicode iterators and indexes
2) added both char* and wchar_t* versions of many wxString methods to avoid having to use _T()

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@45078 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775