add embroidery formats to inkscape export dialog (#2489)

templates/output.xml

@@ -2,7 +2,7 @@
 <inkscape-extension translationdomain="inkstitch" xmlns="http://www.inkscape.org/namespace/inkscape/extension">
     <name>{{ format | upper }} file output</name>
     <id>org.inkstitch.output.{{ format }}</id>
-    <output>
+    <output is_exported="true">
         <extension>.{{ format }}</extension>
         <mimetype>{{ mimetype }}</mimetype>
         <filetypename>Ink/Stitch: {{ description }} (.{{ format }})</filetypename>