Fix ColumnAlignments breakage due to System` prefixes

Fix ColumnAlignments->Center in GridBox producing bogus alignments,
(like \begin{array}{ss} in TeX, or <mtd columnalign="system`center">
in MathML) by checking for them in a dict and substituting the correct
alignment.

Fixes TeX errors building the PDF documentation. Add tests to catch
this kind of issue.