In order to simplify custom components usage by plugins, provide DOM
hooks for plugin-defined custom components. Previously DOM hook API was
only provided for API-generated placeholder elements.
Other changes:
- endpoint insertions now can be performed at any point in time and will
retrospectively attached to DOM for fully initialized endpoints as well
- helper method for querying all hook instances
- getLastAttached() method returns a promise to simplify singleton
element setup and usage (e.g. popups)
Change-Id: I1c95146f76ee52bea3b9e01b666112ce6448efcd