Ignore known but mis-placed elements as if they are unknown elements.

Origin of the issue: https://bugs.documentfoundation.org/show_bug.cgi?id=143600

See if we can treat known but mis-placed elements as if they are unknown elements, and ignore their whole substructures. Ideally, this needs to be handled uniformly across all XML-based import filter code. Also, there should be an option to switch between the strict mode (current behavior) and the "ignore misplaced" mode.