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's user manual
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Contents:

.. toctree::
   :maxdepth: 2

   install
   start
   edit
   build
   connectors
   debug