Fix longpress for Decodable/Leveled Reader (BL-620)

Need to use JohnT's offset method of resetting the cursor
rather than just saving the range as before.

This moves three methods from ReaderToolsModel
to the new EditableDivUtils.
I am testing with lots of brs including after a series of them and between them.  Additionally, I am testing after and between spaces (&nbsp;).  My content doesn't have any <p> tags.  I am not able to reproduce a problem.  I called the method here the same way undo does.
Perhaps I am misunderstanding, though.