Instead of hiding this functionality from users, we instead add an explicit
toggle to the Table Designer dialog "Table" menu to hide or show
the header cells in the designer, and re-add the widgets which
allow users to customise the header appearance back into the
item configuration widget.
Whenever a header cell is selected in the table designer, the formatting
widgets are disabled since the item's header configuration widgets
are used to set the background/foreground color and other properties
of these cells instead.
