Fix lyx2lyx issue with complex beamer arguments.

Fixes: #9254
Fix some convert_TeX_brace_to_Argument glitches

Fixes: #9273
 Fix bug with wrong baseline calculation in last paragraph (2nd attempt)

This time, we consider the cases where a \par is not wanted (single-par insets, tabular cells).

Fixes: #9231
Add support for the pdfcomment package.

Fixes: #6975.
Document pdfcomment in LaTeXConfig.lyx