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.
Merge pull request #106 from andrew-polk/BL620_DRLongPressFix

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