Deep Dungeon redesign

[tukkek]

Now that there are dungeon subtypes and a much better dungeon map generator, the Deep Dungeon could be transformed into something much more interesting than a tiered fight location with Underground maps. One ambitious vision would be to generate a truly huge dungeon floor that keeps its layout but updates its content every time a more powerful Squad enters.

Another idea would be to have ever-expanding dungeon levels, literally bottomless - with each level having a % chance of having an Exit Feature that allows the player to enter directly to that level instead from the outside (choosing from a menu).

This is obviously dependent on #158 to avoid rework.

[tukkek]

Endless Dungeon is probably a better name for this location too.