Introduce map Class field to replace Selectable/UseAsShellmap

This cleans up some uglyness in the current map format and generalizes the map chooser for use by the ingame map editor.
As I already mentioned in IRC:

I think either replacing `Standard` and `Mission` with `Multi-Player` and `Single-Player`, respectively, or splitting `Mission` into `Single-Player Mission` and `Multi-Player Mission` would be an advantage in the future (I'm leaning towards the second solution atm, since that wouldn't require any changes to the maps in case we ever invent a separate lobby for mp missions/campaigns).

Consider the case of an mp mission. The map author would have to choose `Standard` as the class (as opposed to `Mission`, although the map clearly is one). Instead he/she can set the type to something arbitrary. I think changing that would be more intuitive and less inconsistent.

Also, as discussed in IRC, :+1: to renaming `Type` to `GameMode`. Avoids confusion.