Conditional role link (#471)

* Update links demo to include change of projection work

* Add demo by Ahmad / Peter to demo folder. Clean up by moving some demos to
the experiments repo, where they will be publicly usable.

* Clean up demo directory a bit

* Update continuous integration to only execute on pull requests, remove push

* Add role='application' to FeatureGroup <g> element

* Conditionally create role="link" on graphics that are wrapped with <map-a>

* Change role="application" to role="button" on featureGroup, 
which makes more sense when announced

* Add <svg role="none presentation"... so that each feature is not announced
as "Graphic".

* Remove obsolete comment

src/mapml/features/featureGroup.js

@@ -19,8 +19,10 @@ export var FeatureGroup = L.FeatureGroup.extend({
       if(layers.length === 1 && firstLayer.options.link) this.options.link = firstLayer.options.link;
       if(this.options.link){
         M.Feature.prototype.attachLinkHandler.call(this, this.options.group, this.options.link, this.options._leafletLayer);
+        this.options.group.setAttribute('role', 'link');
       } else {
         this.options.group.setAttribute("aria-expanded", "false");
+        this.options.group.setAttribute('role', 'button');
         this.options.onEachFeature(this.options.properties, this);
         this.off("click", this._openPopup);
       }

src/mapml/features/featureRenderer.js

@@ -4,7 +4,25 @@
  * @returns {*}
  */
 export var FeatureRenderer = L.SVG.extend({
+
 
+  /**
+   * Override method of same name from L.SVG, use the this._container property
+   * to set up the role="none presentation" on featureGroupu container,
+   * per this recommendation: 
+   * https://github.com/Maps4HTML/Web-Map-Custom-Element/pull/471#issuecomment-845192246
+   * @private overrides ancestor method so that we have a _container to work with
+   */
+  _initContainer: function () {
+    // call the method we're overriding, per https://leafletjs.com/examples/extending/extending-1-classes.html#methods-of-the-parent-class
+    // note you have to pass 'this' as the first arg
+    L.SVG.prototype._initContainer.call(this);
+    // knowing that the previous method call creates the this._container, we
+    // access it and set the role="none presetation" which suppresses the 
+    // announcement of "Graphic" on each feature focus.
+    this._container.setAttribute('role', 'none presentation');
+  },
+
   /**
    * Creates all the appropriate path elements for a M.Feature
    * @param {M.Feature} layer - The M.Feature that needs paths generated