[Selection] Allow providers to return array

...to allow one-to-many providers for actions, as is useful
for Create et al (and, in this specific case, to support
adapters.)

src/ui/ActionRegistry.js

@@ -6,10 +6,14 @@ define([], function () {
     ActionRegistry.prototype.get = function (context) {
         return this.providers.filter(function (provider) {
             return provider.appliesTo(context);
-        });
+        }).map(function (provider) {
+            return provider.get(context);
+        }).reduce(function (a, b) {
+            return Array.isArray(b) ? a.concat(b) : a.concat([b]);
+        }, []);
     };
 
-    ActionRegistry.prototype.addProvider = function (provider) {
+    ActionRegistry.prototype.register = function (provider) {
         this.providers.push(provider);
     };