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.

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<?xml version="1.0" encoding="UTF-8"?>

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<BeremizRoot URI_location="LOCAL://">

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  <TargetType/>

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</BeremizRoot>