core: Unprepare all backends when destroying the BackendStore
core: Unprepare all backends when destroying the BackendStore

This helps free up some reference cycles (particularly in the BlueZ
backend) and hence ensure no memory is leaked.

https://bugzilla.gnome.org/show_bug.cgi?id=712274

GNOME/folks,GNOME/folks,pwithnall/folks,GNOME/folks,pwithnall/folks,pwithnall/folks