Use a simple implementation working under all OS X versions, including 10.4
which doesn't have standard system images for the close button.
Added the images themselves under art/osx and png2c.py helper script to
convert them to a form used in C++ code.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62300 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
