Warn in GUI when mixing Title and InTitle layouts

Warn in GUI when mixing Title and InTitle layouts

Since we process layouts sequentially, we export LaTeX code for the
title once we arrive to a layout that has InTitle false. If the
document then later has a layout with InTitle true, we do not
(currently) go back to add it to the title and just output it
in-place. We previously warned with LYXERR0, but since this can
cause missing or unexpected output we now warn in the GUI.

For more information, see the following lyx-devel thread:
https://www.mail-archive.com/search?l=mid&q=a65ae226-d3bd-8fc5-a93b-7bb23f1cda82%40lyx.org