- 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

docs/Contributing.md

@@ -7,6 +7,7 @@
   - `Breaking change: `
   - `Fix: `
   - `Fix (<component>): ` Component may be an extension, locale, etc.
+  - `Forward compatibility enhancement: `
   - `Enhancement: `
   - `Refactoring: `
   - `Linting (<type>):` - Linting by type, e.g., "ESLint"