Campaign implementation for Single Player.
In the interest of modulability, provide a basic campaign interface and a common core.
Files in gui/campaigns directly are the common modules (unconvinced about the value of a "common" folder here).
Files in gui/simple_campaign are the files for the "campaign menu" screen, similar to AoE 2's maps where you clicked on a specific scenario. This one is intended to be replaced/extended by basically all campaigns and is rather ugly and limited atm.
Files in gui/simple_setup are the files for the "start a new campaign" screen, to select which campaign you want to start. This one may or may not get extended by mods, but for "pure" campaigns I don't think it'd be necessary.
- I suppose campaign maps should go into their own folder, should that folder be inside the public/campaigns/ folder?
- Where do we store campaign images?