Solve A* in a separate thread

[BarthPaleologue]

Plotting itineraries in the starmap is done using the A* algorithm.

Update steps take O(ln(n)) steps to complete where n is the number of steps already completed (insertion of new nodes inside a binary tree). This can become a problem for distant stars, causing the framerate to drop when looking for the shortest path.

A solution for this is to run the A* solver inside a Web Worker so that the main thread retains a constant framerate.