Share custom XProcSpec assertions x:pef-compare and x:obfl-compare

Move to x:pef-compare to pef-utils and x:obfl-compare to the new
obfl-utils.
Various changes in css-utils

- rename css:eval-content-list to css:parse-content
- rename css:parse-declaration-list to css:parse-properties
- modify css:label-targets so that no two elements will get the same
  @css:id
- modify css:split so that @css:id attributes are removed on middle last
  parts of boxes
- make css:parse-counter-set aware of the automatic 'page' counter
- improve handling of display=none in css:make-boxes
- add option properties-as-attributes to css:new-definition
- add css:shift-id for moving @css:id attributes from non css:box
  elements to the following css:box
Use css:new-definition in css-to-obfl

for parsing/validating/transforming/normalizing properties
fixup 3cf5d090be9163
css-to-obfl: fully support cross-references and TOCs

- evaluate target text, target strings and target 'page' counters
- render leaders
css-to-obfl: add a test for lists

see commit "Better support for counters"
Fix liblouis-mathml tests