use nice and pretty mono font in graphical languages (SFC, FBD, LD) as well

it looks like that was original idea, but some bug squashed in and
default font was used for POU.

at the same time fix use of mono font in the header of Variable panel
for SFC/FBD/LD POUs.

Beremiz's user manual
=====================

Contents:

.. toctree::
   :maxdepth: 2

   install
   start
   edit
   build
   connectors
   debug