Account for largest item in wxGenericListCtrl::GetBestSize().

Account for largest item in wxGenericListCtrl::GetBestSize().

Don't just return a hard-coded value but at least return something big
enough to show the largest item in the control in non-report mode.

This fixes the appearance of wxListbook which simply truncated its items
before if they didn't fit in 80 pixels horizontally.

Also switch to implementing DoGetBestClientSize() instead of DoGetBestSize()
as this method doesn't account for the control borders (it does account for
the scrollbars however).

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@64884 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775