Table header divider lines draw in the wrong place when a table uses `intercellSpacing`

[karlvr]

I believe this is because the header table cells take up the same space as a normal table cell, that is, not including the intercellSpacing, so the AquaTableHeaderCellRenderer renders its AquaTableHeaderBorder but it's inset by the intercellSpacing.

The vertical dividers can easily be drawn in the AquaTableUI... perhaps that's an option? Or I guess if we know we're using AquaTableHeaderCellRenderer then the AquaTableUI could increase the size of the cells to include the intercellSpacing?