Native support for \smash[t] and \smash[b]

Native support for \smash[t] and \smash[b]

This is a follow-up of bug #8967. The implementation is self-explaining, the
only part which needs a comment is lyx2lyx: Since a 100% correct solution is
not possible, it has been decided not to switch amsmath off in the forward
conversion if no other ams command than \smash[t] and \smash[b] is used, but
to consider it a bug that older versions do not load amsmath automatically for
these commands. In the backward direction it is easy to keep the document
compilable, so just do that.