Add wxGrid::DisableHidingColumns() method which can be used to prevent
wxHeaderCtrl from allowing the user to hide columns interactively, which
is something it allows to do by default, unlike the "built-in" wxGrid
header.
Also add EnableHidingColumns() and CanHideColumns() for consistency with
the other similar methods.
Closes https://github.com/wxWidgets/wxWidgets/pull/1554