Make highlighter production-ready

[DivineDominion]

As of now, the yellow highlighter doesn't work in some cases.

• Resizing the window vertically changes textContainerInsets, and the highlighter needs to be moved by the delta of textContainerInset.height - overscrollTopOffset from before and after the update
• Resizing the window horizontally causes a reflow of text and thus moves the insertion point around on screen. The highlighter does not follow
• Make highlighter color configurable