Add accessible carets preference

It will be easier to compare accessible caret with the old
touch/selection carets behavior by toggling preference.
Fix incorrect carets position

The custom content container has "position: fixed" in ua.css. Instead of
transforming caret position to the canvas frame, we transform it to the
frame of custom content container.
Add a red border to caret for debugging
Remove dom prefix in cpp
Add SetAppearance() to unify tilt and visibility