liblouis-formatter: split handle-css-box-model.xsl into smaller parts

... because smaller parts with a simple and specialized API are better
suited for unit-testing and for reuse.

Also, use the new properties `left' and `right' instead of the
"proprietary" `-louis-reset-margin-left' and `-louis-reset-margin-right'
Mark failing XProcSpec tests with pending

also make better use of "$temp-dir" and fix some PEF test files.
Fixup 96a4e0305ca15db5fc5d67bb12b28af03e56e3d4
css-calabash: run tests with Pax-Exam

also mark failing XProcSpec tests with 'pending'
Add CSS properties `left' and `right'
Port liblouis-utils tests to XSpec and move some tests to liblouis-core
Add an XSpec test for css-utils