We were, for some reason, preventing the default behavior of touch
events onTouchStart and onTouchEnd. This made the slider unusable with
drag actions on mobile. I can't remember why we did this to begin with,
probably something important...
The slider thumb size was also increased for better tap target size.
The input slider handle now animates in response to
mousedown/touchstart, and returns to its initial state on
mouseup/touchend. This does not break existing behaviour when navigating
via keyboard.
This is now in compliance with the spec, using a single color with
opacity to render open/hovered/active GridItems and InfoPanels. We also
properly show background on open GridItems.
