Make this a subset of application/zip.

This way, Dolphin will still show Ark etc. as secondary options to 'open
with'.

src/cantor.xml

@@ -4,5 +4,6 @@
     <comment>Cantor worksheet</comment>
     <glob pattern="*.cws"/>
     <generic-icon name="cantor"/>
+    <sub-class-of type="application/zip"/>
   </mime-type>
 </mime-info>