SVGHMI: add support for multiline widget declaration using svg:desc.

Declaration started in inkscape:label and interupted by a '\' can be continued in the description field.
After an empty line, the rest of the description field is left for description text.

.. Beremiz documentation master file

Beremiz's documentation
=======================

Contents:

.. toctree::
   :maxdepth: 2

   overview
   manual/index
   standards