eds: Tidy up the e-d-s Backend.unprepare() method
eds: Tidy up the e-d-s Backend.unprepare() method

This makes sure we disconnect from all the right signals, and abstracts
adding and removing address books out into a single pair of methods.

GNOME/folks,renatofilho/folks,renatofilho/folks2,pwithnall/folks,renatofilho/folks,pwithnall/folks,renatofilho/folks2,GNOME/folks,pwithnall/folks,renatofilho/folks,renatofilho/folks2,renatofilho/folks2,GNOME/folks,renatofilho/folks