Add option to limit output extension to save copy
The intention of an extension author might be to limit their output extension to just being available in the "Save a Copy" menu, doubly enforcing the existing dataloss flag.
<inkscape-extension xmlns="http://www.inkscape.org/namespace/inkscape/extension">
...
<output>
...
<savecopyonly>true</savecopyonly>
</output>
</inkscape-extension>