FIX0rzdzca: sort scroller vs. given items:

Accept a scroller or a set of items, given:
* You can't have both
* You can't have none
* Always check for existing scroller with predicate
* Check isa using Safe::Isa

Also:
* Clean up dead code
* Update examples
* Update tests (use better fake Scroller object)

OMG MICKEY DID SO MUCH WORK ON THIS THANK YOU