Queue changes until the map becomes visible

[m-r-c]

Add a mechanism to queue all requested changes to the map when the map is not visible.

The plugin currently redraws the map once it becomes visible, which works well in most cases. However, if data is provided to the map when it is not visible, the redraw does not always work.

The logic would be:

1. When addLayer, addMarker, setGeoJSON etc etc are called
2. if the map is visible, make the change
3. if the map is not visible, add the change to a queue of some sort
4. when the map becomes visible, execute all changes from the queu