`<tbody/>` is automatically inserted, so we can guarantee that this affects all ths and tds in the last row of the table *body* and the head is left untouched

move the slide-number out of the controls to a proper position and little fix to the updating event.

the *.min.js didn't chaged yet.

Closes #662.

css/theme/template/settings.scss adds the $headingMargin variable, set to `0 0 20px 0`.
css/theme/template/theme.scss uses $headingMargin to set margin on headers.

They looked a little wonky if you used a G or a Y.

The Serif theme was a little flaky previously