Editor: Image overlay for maps

[kaputtnik]

Suggested in the forum: https://www.widelands.org/forum/topic/4600/

> How do the devs see the possibility to have a picture
> overlay feature implemented?
> I would like to layout a map in an Image Editor and use
> this as overlay in the game editor. That would help to
> make maps that are more symmetric or even based on
> satellite images of real places.

I think this would be great feature.

[GunChleoc (gunchleoc)]

Migrated to https://github.com/widelands/widelands/issues/2147