Use pp2's facet names ("xtargets", "subject", "author") instead of

Use pp2's facet names ("xtargets", "subject", "author") instead of
	unnecessary MKWS-specific variants ("sources", "subjects",
	"authors").
The pp2 object is now initialised to fetch only the facets that we're
	actually going to use, rather than blinding fetching all three
	whether they're used or not.
No "onterm" callback function is provided when no facets at all are
	required, so that PP2 is never asked for them. This is going
	to be a significant optimisation when using multiple Fresco
	widgets.
my_onterm() now raises an alert on the "can't happen" condition that
	we are invoked when no facets were requested. This defensive
	clause can be removed later.