Refactor Embed API

[simboss]

The current embed API uses iFrames in HTML. This is considered a security threat in many environments hence we should go beyond this approach and introduce a more modern approach.

[simboss]

• possibility capture events Add notification of actions and state changes to the js api #1619
• possibility to pass a map context
• possibility to load a map from MapStore 2 back-end API: add the ability to load configuration from a URL #1624
• clear documentation for the APIs Document javascript api #1549