Improve Cesium map ready notification to EMP core.

[alberto-acevedo]

The Cesium map has no way to notify when the map is ready. This is why there is a kind of long timeout delay when initializing the map. A better way to detect when the map is ready is by using the already available post render event listener. A map is ready notification is triggered the first time this event listens to a map post render. A short timeout time is still used for synchronization purposes with the core.

[alberto-acevedo]

Already merged into the dev branch. Fixed on #274.