Allow to customize wxGrid column auto-sizing.

By default the columns are auto-sized to fit just their label, which is fast
but not very user-friendly. Allow customizing this behaviour by handling the
(new) wxEVT_GRID_COL_AUTO_SIZE event.

Closes #15077.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73789 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775