Make it possible to add or remove change actions to overflow menu

``` js
Gerrit.install(function(plugin) {
  // Move cherry-pick button out of overflow menu.
  plugin.setActionOverflow('revision', 'cherrypick', false);
  // Move submit button out of overflow menu.
  plugin.setActionOverflow('revision', 'submit', true);
});
```

Revision and change actions are as returned by Gerrit's REST API.

Feature: Issue 5360
Change-Id: I151894b39929bd67ef0e00802c699831ab3f72fc

polygerrit-ui/app/elements/shared/gr-dropdown/gr-dropdown.js

@@ -23,6 +23,12 @@
      * @event tap-item-<id>
      */
 
+    /**
+     * Fired when a non-link dropdown item is tapped.
+     *
+     * @event tap-item
+     */
+
     properties: {
       items: Array,
       topContent: Object,
@@ -93,7 +99,13 @@
 
     _handleItemTap: function(e) {
       var id = e.target.getAttribute('data-id');
+      var item = this.items.find(function(item) {
+        return item.id === id;
+      });
       if (id && this.disabledIds.indexOf(id) === -1) {
+        if (item) {
+          this.dispatchEvent(new CustomEvent('tap-item', {detail: item}));
+        }
         this.dispatchEvent(new CustomEvent('tap-item-' + id));
       }
     },

polygerrit-ui/app/elements/shared/gr-dropdown/gr-dropdown_test.html

@@ -74,13 +74,17 @@ limitations under the License.
     });
 
     test('non link items', function() {
-      element.items = [
-          {name: 'item one', id: 'foo'}, {name: 'item two', id: 'bar'}];
-      var stub = sinon.stub();
-      element.addEventListener('tap-item-foo', stub);
+      var item0 = {name: 'item one', id: 'foo'};
+      element.items = [item0, {name: 'item two', id: 'bar'}];
+      var fooTapped = sinon.stub();
+      var tapped = sinon.stub();
+      element.addEventListener('tap-item-foo', fooTapped);
+      element.addEventListener('tap-item', tapped);
       flushAsynchronousOperations();
       MockInteractions.tap(element.$$('.itemAction'));
-      assert.isTrue(stub.called);
+      assert.isTrue(fooTapped.called);
+      assert.isTrue(tapped.called);
+      assert.deepEqual(tapped.lastCall.args[0].detail, item0);
     });
 
     test('disabled non link item', function() {
@@ -88,10 +92,13 @@ limitations under the License.
       element.disabledIds = ['foo'];
 
       var stub = sinon.stub();
+      var tapped = sinon.stub();
       element.addEventListener('tap-item-foo', stub);
+      element.addEventListener('tap-item', tapped);
       flushAsynchronousOperations();
       MockInteractions.tap(element.$$('.itemAction'));
       assert.isFalse(stub.called);
+      assert.isFalse(tapped.called);
     });
   });
 </script>