- Imagelib backward compatibility enhancement: Allow string based API

again so as not to break old SVG-Edit which fail at *presence* of
  `namespace` (fixes #274)
- Known regression: Remove Openclipart as its site's now setting of
  `X-Frame-Options` to `"sameorigin"` makes it unusable on our end
  for our cross-origin uses
- Forward compatibility enhancement: For IAN image library, add
  `svgedit=3` param to URL so that it can keep using old API for
  SVG-Edit versions before 3, while conditionally using new object-based
  API now (and if we switch exclusively to the object-based API in the
  future, this site will continue to work)
- Docs: Add "Forward compatibility enhancement" to list

editor/extensions/ext-locale/imagelib/sl.js

@@ -17,13 +17,17 @@ export default {
     },
     {
       name: 'IAN Symbol Libraries',
-      url: 'https://ian.umces.edu/symbols/catalog/svgedit/album_chooser.php',
+      url: 'https://ian.umces.edu/symbols/catalog/svgedit/album_chooser.php?svgedit=3',
       description: 'Free library of illustrations'
-    },
+    }
+    /*
+    // See message in "en" locale as to why disabling
+    ,
     {
       name: 'Openclipart',
       url: 'https://openclipart.org/svgedit',
       description: 'Share and Use Images. Over 50,000 Public Domain SVG Images and Growing.'
     }
+    */
   ]
 };