For large dataview controls, don't use all items to calculate best column width.

For large dataview controls, don't use all items to calculate best column width.

Instead, use just top and bottom N/2 items for some large enough value
of N. N is determined dynamically so that column best width calculation
doesn't take more than 50ms.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65951 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775