A whole bunch of changes to do facets properly:

* We no longer keep mkws.pp2filter state-variable around
* limitTarget() and unlimitTarge() no longer mess around with Pazpar2
  filter syntax
* limitQuery() no longer messes with the actual query
* triggerSearch () now calculates pp2filter (for targets) and pp2limit
  (for field values) on the fly from mkws.filters.
* Limits as well as filters are passed into my_paz.search()
* limitQuery() calls resetPage() etc. by hand, as its sibling
  functions do, instead of delegating to onFormSubmitEventHandler()
* Pass pazpar2 fieldnames rather than CCL indexes into
  add_single_facet()

All of this fixes bug MKWS-63 ("Reimplement facets to use Pazpar2
filtering rather than query manipulation")