Add a <cxx-include> element to allow building a TS out of multiple files.

I may need to fix up the <cxx-clause> numbering to get included clauses to
number in the right order.
Update clause numbers through the ToC
to allow new clauses to be added dynamically in the middle of the document.
Turn images into data: URLs when publishing.
Add a <cxx-figure> element analogous to <cxx-table>.

Also made tables renumber themselves when they're added or removed from the
document.  This isn't as efficient as it could be, but I can optimize that
later.
Add <cxx-preconditions> element.
Give references a title (tooltip) of the symbolic name of the target.
Teach <cxx-ref> to watch for new elements being added, which might have a needed ID.
Teach <cxx-section> to check for an id, and document the checkInvariants() function.