Display the correct horizontal alignment in AMS environments

Display the correct horizontal alignment in AMS environments

A longstanding problem... (related: #1861)

The columns in AMS math environments have a fixed alignment (colAlign() in
InsetMathGrid.cpp). We set this alignment for display (Georg's
displayColAlign()) in InsetMathHull and InsetMathSplit. This is done according
to tests and documentation for the various environments.

There is also some mechanical code factoring via colAlign().

Finally, I disable setting the horizontal alignment in InsetMathSplit, which has
no impact on the LaTeX output, and has no longer any impact on the screen. (As
for vertical alignment I discovered that it was in fact customisable for
\aligned & friends! I hope that the more faithful interface will let other
users discover that too.)