Kindle APNX: Add calculation using pagebreak tag.

I don't have a kindle so I have not tested this code.

lp bug #1321761. Instead of overloading the exact page count parameter it's a separate function. The kindle can be configured to use this method as one of the "accurate" calculation methods.

Right now I have the method as a text box because it doesn't appear that the device config widget allows lists of values to be displayed in a combo box. Something for the future unless I misread it.