[Toolbar] Implement a public API for adding toolbars (#1908)

* [API] Implement a toolbar registry and a plugin to allow providing a toolbar for a selected object.
* Modify the mct-toolbar directive to get the toolbar structure from a provider based on selection.
* Implements the layout toolbar in the layout bundle

platform/commonUI/general/src/directives/MCTSelectable.js

@@ -28,6 +28,16 @@ define(
          * The mct-selectable directive allows selection functionality
          * (click) to be attached to specific elements.
          *
+         * Example of how to use the directive:
+         *
+         *   mct-selectable="{
+         *       // item is an optional domain object.
+         *       item: domainObject,
+         *       // Can define other arbitrary properties.
+         *       elementProxy: element,
+         *       controller: fixedController
+         *  }"
+         *
          * @memberof platform/commonUI/general
          * @constructor
          */