Story mode map system

[smerkli]

The current way to start a level is via the level selection screen. Even though this will always be a possibility in the game, a story mode is still missing. The idea would be to implement some sort of map on which one could click on the next mission - possibly with a mission briefing screen or similar.

This might be a two-person job as well - someone could work on the briefing screen and the second person on the clickable map.