Add a move constructor and move assignment operator for
o3tl::cow_wrapper. Insubstantial gains for UnsafeRefCountingPolicy, no
atomic increment needed for ThreadSafeRefCountingPolicy's move-ctor.

Change-Id: Ia2de1ca78b1e0e5a0f30535e752f1dd858fdfef0
Reviewed-on: https://gerrit.libreoffice.org/17848Reviewed-by: Thorsten Behrens <Thorsten.Behrens@CIB.de>
Tested-by: Thorsten Behrens <Thorsten.Behrens@CIB.de>