Add a new icon for disabling cell highlight - not white

The 'white' colour in the "select colour" dropdown for cell highlighting doesn't actually mean "make this white". Rather, it means "turn off highlighting". In a normal cell, this is the same as making it white, but in a header row it returns the cell to the standard grey. This can be confusing.

We need to add a new icon. For example, it could show a white square with red line through it.