Doomsday Engine

Added (All games): Darken the screen after the users quits from the menu.

A quick new feature to distract from the monotony of the current work in mapcache2. Interestingly this has revealed a couple of issues that we should consider:

• When not quiting immediately the user is still able to open the console and execute commands. This is fine, we don't want to deprive the user from accessing the console. However many game-side commands should not be executed once game shutdown has begun. Adding a "is shutting down" test to each command seems clumsy.
• Using a gamestate for "a map is loaded" is rather awkward and will need a rethink.