PDImage also caches rating property in ivar
added commands for ratings-predicates to image-list search field
rework how image-list predicates are applied

	* if no predicate is set, do equivalent of "rating >= 0", i.e.
	hide rejected images

	* to stop this breaking the trash query, add a new property to
	PDLibraryQuery: BOOL nilPredicateIncludesRejected. The trash
	item sets it to true

	* when an image property of a selected image changes that means
	it no longer matches the active predicate, preserve the image
	on screen until it's no longer selected
added commands to move the library view selection
scroll library view selection to make it visible when it changes