Implement cheats

[TheJJ]

As a very important feature, we need cheats of course.

Implement those in the libopenage/cheat subsystem, which is then used by the engine to trigger all the funny actions.

[timo-42]

you should have a look at #649 . I describe triggers which have the same context as cheats. Some condition => action. Cheat: condition="text in chat" => action