EBookClientCursor: Keep a strong reference to the client

EBookClientCursor: Keep a strong reference to the client

When finalizing a cursor, it is important to have access
to the direct backend available so that it can be asked
to delete the direct access cursor instance.

For this reason EBookClientCursor must keep the EBookClient alive,
EBookClient now exposes a hidden API (not in the header file)
to allow the EBookClientCursor to ask the EBookClient to delete
the cursor on the EBookClientCursor's behalf.

Ugly, but works.