Add 256x256 icon to m17n.appdata.xml

Author: mike-fabian

m17n.appdata.xml

@@ -22,7 +22,8 @@
       <caption>ibus-m17n screenshot</caption>
     </screenshot>
   </screenshots>
-  <icon type="remote" width="128" height="128">https://raw.githubusercontent.com/ibus/ibus-m17n/main/icons/ibus-m17n.png</icon>
+  <icon type="remote" width="128" height="128">https://raw.githubusercontent.com/ibus/ibus-m17n/main/icons/128x128/ibus-m17n.png</icon>
+  <icon type="remote" width="256" height="256">https://raw.githubusercontent.com/ibus/ibus-m17n/main/icons/256x256/ibus-m17n.png</icon>
   <project_license>GPL-2</project_license>
   <developer_name>Parag Nemade, Mike FABIAN</developer_name>
   <update_contact>pnemade@redhat.com, mfabian@redhat.com</update_contact>