We want to support things like

<tscreen><verb>
I am a <tt/verbatim/ line
</verb>
I am not a <tt/verbatim/ line
</tscreen>

Since nsgmls will break lines when look changing commands are found in
tscreen+non-verbatim, we need to make sure that LyX-Code commands are
not sent neither during the look change nor in the line after it. No
problem in verbatim+tscreen, since lines will not be broken.

We use $inlookchange to signal this, 0 or undefined means no look change,
1 means in look change and 2 means last line ended a look change.