Revert OperationFactory interface changes made in a1f86ac

By specifically invoking a new when creating an operation instance
makes it quite difficult for external operation constructors, as there
is no easy way for external ops to share non-document state.

All WebODF ops don't have this issue as new functions can easily be
added to the OdtDocument interface to support the new requirements.
Merge pull request #464 from peitschie/opfactory-tweak

Revert OperationFactory interface changes made in a1f86ac