Experiment removing off-screen maps

[leopiccionia]

MapboxGL instances contain the Map.remove() method that, according to documentation:

Clean up and release all internal resources associated with this map.

This includes DOM elements, event bindings, web workers, and WebGL resources.

Use this method when you are done using the map and wish to ensure that it no longer consumes browser resources. Afterwards, you must not call any other methods on the map.

We should experiment with removing off-screen maps to save resource, specially WebGL-related resources.

While removing a map is simple, re-initializing a map to its previous state (including interactions, etc.) when it's back to screen may require non-trivial refactoring.