liblouis-formatter: refactoring

- extract handle-css-content.xsl from handle-css-before-after.xsl
- use internal attribute css:display
- ...
liblouis-formatter: more refactoring

- introduce css:block and css:space for easier reasoning about
  significant text and whitespace
  - handle-xml-space.xsl wraps text nodes in louis:space if
    xml:space="preserve"
  - identify-blocks.xsl wraps blobs of significant test in css:block
- don't remove CSS properties that don't apply to an element until
  all "inherit" values on descendant elements have been concretized
- move flattening of elements with @css:target="true" to
  normalize-space.xsl
- rename style attribute on louis:border to "pattern"
css-utils: extend eval-content-list

... with support for target-text(), target-string(), target-counter()
and leader()
Deprecate "display: toc-item"

Rely on target-text(), target-string(), target-counter() and leader()
for TOC generation
Improve pef:compare
css-core: deprecate toc-item
bugfix in LiblouisJnaImpl: JNA callback function was being garbage collected
css-core: allow functions in 'content' declarations
liblouis-formatter: render TOC items more accurately

- sometimes disable braille and/or print page numbers in TOCs
- use the correct line fill symbol
xml-to-pef: add a TOC test