Various additions

- New `app.sandbox.ready()` which returns a Promise to allow extensions, or components to be notified when all components are started.

Used for a parent component to ensure its children components have been started, before the parent emits component events to its children.

- Allow `app.sandbox.start` to start the top level component.

Before, it uses `$.find` to search for components to start up, so if the top-level element is an aura component, that component will not be started. Now it will use both `$.find` and `$.filter`.