Refactor the zoom page

Make the design more flexible to sliderHandleWidth, sliderHandleHeight
and sliderGapWidth changes.

Change-Id: Idd5dd07c1951fe70320f8434e0cfe8f211518277
Reviewed-by: Andrew Knight <andrew.knight@digia.com>
Make the height of the row delegate an application constant

Since it is used at different places.

Change-Id: I9cecc118871d948f0cabd80d35cb07a12ccd1b88
Reviewed-by: Andrew Knight <andrew.knight@digia.com>