Plugin Support

The UI must be able to support plugins.

A plugin consists of a static UI component which is able to interface with the main Gentics Mesh UI.
The plugin data will be served by the Gentics Mesh server. These static components will be embedded via IFrames.

Plugins can interface with the Gentics Mesh UI in various ways/places:
* A plugin can provide a dedicated UI via a custom *menu button*
* A plugin can specify *trigger points* which allow for custom dialogs
* A plugin can specify custom *form controls*

Relates to https://github.com/gentics/mesh/issues/323

Start by reviewing the Gentics Mesh UI entry points for the custom menu button, trigger points, and form controls, along with the related Gentics Mesh issue #323. Define how static plugin components served by the Gentics Mesh server are embedded through IFrames and how they interface with the UI; done means all three plugin integration points have an agreed, working design.

Written by the indexing model from the issue text.