Remove updateInfo() calls in favor of doing the relevant work

Remove updateInfo() calls in favor of doing the relevant work
in updateBuffer().

This is in response to a reported crash. See
	https://www.mail-archive.com/lyx-devel@lists.lyx.org/msg202217.html
Commit af381a2f addressed the crash, and is worth doing anyway. But
this also makes sense.

Also adds a flag to tell us whether we need to recalculate the
relevant information. In some cases, there is no need to do so
more than once. (Note that, with the existing code, we *never*
recalculate, which would have given rise to bugs.)