Change alignment of inline equations (non-MathJax).

This is better for users of screen-magnifiers. Centered equations will
be difficult to find. A fixed distance from the left margin is better.
Rename table qtype_stack to qtype_stack_options.

Sadly, we started before the convention was firmly established, and then
the naming convention went the other way.
Front-end settings for how to display inverse trig functions.

Chris will need to link these up to the CAS code to make them work, but
there is a new field qtype_stack_options.inversetrig with three possible
values cos-1, acos or arccos.

This can be set on the editing form, and is correctly loaded and saved,
imported and exported, backed up and restored.
Fix issues with questions that have no PRTs and/or no inputs.

A STACK question with no inputs now behaves exactly like a Moodle
'Description' question.

A STACK question with inputs but no PRTs behaves like a Moodle Essay
question. If it is graded out of 0, then it has no effect on the score
at all. If it has a non-zero max mark, then it can be manually graded,
and the quiz total score is not computed until it has been.

Note that the manual graded case really ought to use a special
maualgradedwithexplicitvalidation behaviour, but life is too short, and
everyone should be using instant validation anyway.