[doc] Document how to use SVG icons for EMF metamodels

Signed-off-by: Pierre-Charles David <pierre-charles.david@obeo.fr>

doc/how-to/use-svg-icons-in-emf.adoc

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+= How to use SVG icons for EMF metamodels
+
+By default, the code generated by EMF for its metamodels only support bitmap formats for the element icons.
+This is not adapted to modern contexts, especially on the web.
+It is possible to force EMF to use SVG icons but this requires a few tweaks.
+
+1. In the main `EditPlugin` class in that project (the one which extends `EMFPlugin`), add the following code to override the default `doGetImage` implementation:
+[source,java]
+----
+@Override
+protected Object doGetImage(String key) throws IOException {
+    URL url = new URL(this.getBaseURL() + "icons/" + key + extensionFor(key));
+    InputStream inputStream = url.openStream();
+    inputStream.close();
+    return url;
+}
+
+/**
+  * Computes the file extension to be used with the key to specify an image resource.
+  *
+  * @param key
+  *            the key for the imagine.
+  * @return the file extension to be used with the key to specify an image resource.
+  */
+protected static String extensionFor(String key) {
+    String result = ".gif";
+    int index = key.lastIndexOf('.');
+    if (index != -1) {
+        String extension = key.substring(index + 1);
+        if ("png".equalsIgnoreCase(extension) ||
+            "gif".equalsIgnoreCase(extension) ||
+            "bmp".equalsIgnoreCase(extension) ||
+            "ico".equalsIgnoreCase(extension) ||
+            "jpg".equalsIgnoreCase(extension) ||
+            "jpeg".equalsIgnoreCase(extension) ||
+            "tif".equalsIgnoreCase(extension) ||
+            "tiff".equalsIgnoreCase(extension) ||
+            "svg".equalsIgnoreCase(extension)) {
+            result = "";
+        }
+    }
+    return result;
+}
+----
+The only difference with the default/normal implementation is that the `extensionFor` method used now knows about the `.svg` file extension.
+2. Place the icon(s) you want for you types as SVG files in the `icons/full/obj16` folder of the `.edit` generated plug-in/project.
+3. For each type with such an SVG icon, modify its `ItemProvider`'s `getImage` method to reference the actual SVG image path, *including the file extension*:
+[source,java]
+----
+/**
+  * @generated NOT
+  */
+@Override
+public Object getImage(Object object) {
+    return this.overlayImage(object, this.getResourceLocator().getImage("full/obj16/SliderDescription.svg"));
+}
+----
+Do not forget to mark the method as `@generated NOT` so the customization is not lost on regeneration.