Allow disabling hiding columns when using wxHeaderCtrl in wxGrid

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

include/wx/generic/private/grid.h

@@ -147,7 +147,7 @@ public:
                        wxID_ANY,
                        wxDefaultPosition,
                        wxDefaultSize,
-                       wxHD_ALLOW_HIDE |
+                       (owner->CanHideColumns() ? wxHD_ALLOW_HIDE : 0) |
                        (owner->CanDragColMove() ? wxHD_ALLOW_REORDER : 0))
     {
     }