use CodeMirror's `setBookmark` to display other user's cursors
use CodeMirror's `setBookmark` to display other user's cursors

the advantage of this approach is that the cursor stays at the right
position of the text automatically. One disadvantage is that the cursor
can be removed by selecting the text around the other user's cursor and
pressing backspace.

pingyuanChen/ot.js,Operational-Transformation/ot.js,Operational-Transformation/ot.js,abhishekgahlot/ot.js,pingyuanChen/ot.js,abhishekgahlot/ot.js,rike422/operatta-js,abhishekgahlot/ot.js,Operational-Transformation/ot.js,pingyuanChen/ot.js