only provide ptrdiff_t versions of verious operator+/- working with iterators instead...

only provide ptrdiff_t versions of verious operator+/- working with iterators instead of overloads for both int and size_t: this seems to be enough and the existing overloads were not enough for 64 bit builds where expressions such as iter+(ptr2-ptr1) didn't compile without extra casts

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@51053 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775