Add support for semantics annotations to toMathML, and use a menu item to control it.
Fix bounding box problem in Firefox with stretchy delimiters (and remove alignment hack that doesn't seem to be needed any longer).  resolves issue #729.
Fix several problems with stretchy delimiters in SVG output (minsize didn't always get processed, stretched characters weren't marked as such).
Don't add stretchy=dalse if it is not needed
Merge branch 'issue814' into v2.4-beta. (semantic annotations in
toMathML output).  Issue #814.
Merge branch 'issue729' into v2.4-beta.  Issue #729.
Merge branch 'stretchy' into v2.4-beta