wxDataViewCtrl: always update the header when col best width changes.

wxDataViewCtrl: always update the header when col best width changes.

Have an explicit per-column dirty flag and use that to determine whether
we need to call wxHeaderCtrl::UpdateColumn(). Previously, the lack of
computed best width was used as an indicator, but this didn't work
correctly if some code called GetWidth() after invalidation but before
wxDataViewCtrl::UpdateColWidths() was called at idle time. This resulted
in header's column widths getting out of sync with the control itself.

Fixes #14167.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@71335 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775