Remove explicit casts to (const char *), and replace it with .c_str();

  Remove explicit casts to (const char *), and replace it with .c_str();
use wxConstCast instead of explicit casts to char*.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@19577 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775