Make it possible to customize the appearance of wxGrid corner window and its
row and column headers by defining custom renderers for them.
Add demonstration of this new feature to the grid sample and update the
documentation.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@61919 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775