Add ability to preview fog of war

[BlackYps]

Normally maps are played with fog of war on. That means that in practice most of the map will look significantly darker. To easily preview this the map editor could offer a toggle to replicate this.
Ideally we would leave holes in the fog of war at the spawn locations. So you can have a direct experience of how it would look in the game, when you see parts of the map exposed and it also serves as a reminder that the setting is still on.