Introduce max_length parameter for plaintext() output routines,

so we can write a limited amount when using this for TOC and
tooltip output.

This should solve the problem with slowness that Kornel noticed,
which was caused by our trying to write an entire plaintext
bibliography every time we updated the TOC. We did that because
he had a bibliography inside a branch, and we use plaintext for
creating the tooltip that goes with the branch list.

Other related bugs were fixed along the way. E.g., it turns out
that, if someone had an InsetInclude inside a branch, then we would
have been writing a *plaintext file* for that inset every time we
updated the TOC. I wonder if some of the other reports of slowness
we have received might be due to this kind of issue?