optimization hack: seek w/ ghost cursor not in DOM

Superseded by cc7652cf48a231c8034dd269ded1cb72500364a1 from #142, PR'd to archive:

instead of repeated DOM manipulation moving the cursor span around, move
a "ghost cursor" around in the edit tree and guesstimate the coords of
the cursor if it were inserted where the ghost cursor is

experimental optimization for #154

this is good motivation for an intermediate class between Point (from
the dev branch) and Cursor