- 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

dist/extensions/ext-locale/imagelib/en.js

@@ -16,12 +16,18 @@ var svgEditorExtensionLocale_imagelib_en = (function () {
       description: 'Demonstration library for SVG-edit on this server'
     }, {
       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'
-    }, {
-      name: 'Openclipart',
-      url: 'https://openclipart.org/svgedit',
-      description: 'Share and Use Images. Over 50,000 Public Domain SVG Images and Growing.'
+      /* ,
+      // Disabling until X-Frame-Options may be removed again (or allow cross-origin):
+      // Getting messages like this in console:
+      // Refused to display 'https://openclipart.org/detail/307176/sign-bike' in a frame
+      // because it set 'X-Frame-Options' to 'sameorigin'.
+      {
+        name: 'Openclipart',
+        url: 'https://openclipart.org/svgedit',
+        description: 'Share and Use Images. Over 50,000 Public Domain SVG Images and Growing.'
+      } */
     }]
   };