Adds -intrinsicContentSize for AutoLayout

Implemented -intrinsicContentSize for proper control sizing with AutoLayout.

This change specifies a minimum height for the control so that it can be used with AutoLayout without specifying dimensions in advance. The exact value can be tweaked since this would result in a very tight fit, but the value provided (the height of the sliders) should be the absolute minimum height required. Prior to this fix the control would appear full-size but the touchable area could be too small to use.