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.

     1

#State Text Reference;;;;;;;

     2

#Reference Number;Text 1;Text 2;Text 3;Text 4;Text 5;...;Text n