This id corresponds to the close button bitmap. Currently only wxGTK returns a
natively-looking button from here, MSW and OS X versions will be added later.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62282 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
