EBookClientView cleanups.

EBookClientView cleanups.

Let EBookClientView create its own D-Bus proxy object by implementing
GInitableIface and adding "connection" and "object-path" construct-only
GObject properties.

This eliminates e-book-client-view-private.h and adds the following
public accessor functions:

   e_book_client_view_get_connection()
   e_book_client_view_get_object_path()