Fix for issue #45

New keyboard shortcuts for moving slider are ctrl comma and ctrl period.

N.B. additional ARIA accessibility label required:

_accessibility._ariaLabels.sliderScaleMarkerPrefix, e.g:

"Slider value "

_accessibility._ariaLabels.sliderHandle text should be updated
accordingly to:

"Slider handle. Hold down the CTRL key and use the comma and full stop
keys to move the slider left and right to give your answer. Then tab to
the submit button below and press enter."