Added wxART_CLOSE art id.

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

interface/wx/artprov.h

@@ -108,6 +108,7 @@ typedef wxString wxArtID;
      @li wxART_PASTE
      @li wxART_UNDO
      @li wxART_REDO
+     @li wxART_CLOSE
      @li wxART_QUIT
      @li wxART_FIND
      @li wxART_FIND_AND_REPLACE