Support multiple areas

[JohannesLamberts]

as the density of a crows is often not uniformly it would be helpful to add multiple polygons to the same map with different densities.
Opening the tool in multiple tabs is difficult to manage and its almost impossible to handle intersections.

I'd be open to contribute if you could add a License to the repo.

[paraboul]

Hey @JohannesLamberts, that would be interesting indeed!
The hardest part here is to keep the UI as simple as possible