add support for a spacer element within cards

Author: frett

public/xmlns/tract.xsd

@@ -141,6 +141,7 @@
                     text-scale: DEFAULT( 1.0 )
             -->
             <xs:choice minOccurs="0" maxOccurs="unbounded">
+                <xs:element ref="content:spacer" />
                 <xs:element ref="content:paragraph" />
                 <xs:element ref="content:form" />
             </xs:choice>