split string.{h,cpp} into {string,stringimpl,arrstr}.{h,cpp} to make the files more managable

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@45098 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/unichar.h

@@ -13,10 +13,7 @@
 
 #include "wx/defs.h"
 #include "wx/chartype.h"
-
-// NB: this header is included from string.h as well, but from the place
-//     where wxStringImpl is already declared and that's all we need
-#include "wx/string.h"
+#include "wx/stringimpl.h"
 
 class WXDLLIMPEXP_BASE wxUniCharRef;