Fix fixed-width progress columns in generic wxDataViewCtrl.

Both native wxDVC implementations grow the progress column, the generic
one didn't.  This change fixes it and _seems_ not to break other things.
As a consequence, progress columns no longer have a 40px minimal width,
but that should be OK.

Fixes #15745.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@75346 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775