Prevent 2.4 from crashing when loading a book that was opened with 2.6 before.

The core issue is that open reports are using a new restore function in 2.6,
which doesn't exist in 2.4 and older. The restore code now checks for the
existence of this new function and falls back to the old one if necessary.
The (mild) side effect is that in 2.4 any link to a custom template is lost.

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23159 57a11ea4-9604-0410-9ed3-97b8803252fd