[LyX/master] Support the mathbbm font.

[Pavel Sanda]

On Mon, Jul 27, 2020 at 04:56:48AM +0200, Thibaut Cuvelier wrote:
> commit e709a6626e731b460a98ea34d762a776de90b54c
> Author: Thibaut Cuvelier <tcuvelier at lyx.org>
> Date:   Mon Jul 27 03:14:38 2020 +0200
> 
>     Support the mathbbm font.
>     
>     Should just be able to read formulae, not add it from the GUI, as it's mostly redundant with mathds and mathbb.

Hi Thibaut,

why is this font addition needed?
And how is it related to the changes in src/output_docbook.cpp?

Pavel