Create a new wxStringBase class, providing a STL-like API, using

  Create a new wxStringBase class, providing a STL-like API, using
the current wxString implementation.
  Derive wxString from one of wxStringBase, std::string, std::wstring
depending on wxUSE_STL/wxUSE_UNICODE settings. wxString provides
full compatibility with the old wxString in both wxUSE_STL=0 and
wxUSE_STL=1 modes (except for [Un]getWriteBuffer).
  Add some configure checks for std::string features.
  Add tests for STL API in console sample.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@22280 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775