EDataBook: Watch the system bus for locale notifications

When org.freedesktop.locale1 is available, listen to changes in
the LC_COLLATE locale and configure backends with locale changes
using e_book_backend_set_locale(), notify property changes via
the locale property on the addressbook D-Bus API.

Also, load the backend's initially set locale as the locale property
value until the org.freedesktop.locale1 D-Bus interface notifies
us of a locale change on the system bus.