This works everywhere right now, but it's obviously not going to stay
that way forever. :^)
Note that this does not advance the cursor correctly for whitespace
since the cursor is DOM-based and doesn't take whitespace collapsing
into account yet.
Each Web::Frame now has a cursor that sits at a DOM::Position. It will
blink and look like a nice regular text cursor.
It doesn't really do anything yet, but it will eventually.
