fdo#77209: Share string pool with clip documents.

We do the same with undo documents, and it will only make sense to do
the same with clip documents as well.  Also, put the sharing part into
a common method (for ease of tracking).

(cherry picked from commit 8f403051968298fbabd61de82fbb6a77762c83cc)

Conflicts:
	sc/inc/document.hxx

Change-Id: I342b22d95374ee06d16318a66ffea0ac5b42621c
Reviewed-on: https://gerrit.libreoffice.org/9005
Reviewed-by: Eike Rathke <erack@redhat.com>
Tested-by: Eike Rathke <erack@redhat.com>