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.
<xsd:element name="Xenomai">
<xsd:complexType>
%(toolchain_gcc)s
<xsd:attribute name="XenoConfig" type="xsd:string" use="optional" default="xeno-config"/>
</xsd:complexType>
</xsd:element>