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.
Are you sure you don't need code like that in doKeypressMarkup() to figure out and use the divBrCount and atStart? For example, do you get the right result doing long press on the blank line at the end of a box, or in the middle of a sequence of blank lines?

You might also need to test in an older book...the <br> problem doesn't happen in some books because they're using a mode that does everything with <p> elements. John wants to go that way, but I don't think everything does yet. See whether the book you're testing on actually has <br> elements in the text.