Separate row/column highlight from the new background colouring

The current UX in Confluence 4.3 to apply headings and cell/row/column highlighting can be confusing.

This change will separate the setting of a column/row to a heading, and colour cell/selection.

It is expected that the table markup should be improved (i.e. actual TH for all cells in a heading column).

To be confirmed if this will be available in a 4.3.x release (this will depend on the scope of the changes).