wxString(const wxString& str, size_t position, size_t len) ctor now doesn't

wxString(const wxString& str, size_t position, size_t len) ctor now doesn't
have default value for the third param to avoid conflicts with
wxString(const wxString& str, size_t len) ctor. Sorry if this breaks code
which relied on it...

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@1305 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775