Add showcase demo games and demo data

[hhirsch]

Add some very basic demo games and demo data to radix as well as a target to build them.

• [ ] Decide on where to put these in the file system and what repositories to create
• [ ] Create repositories
• [ ] Create FPS demo #67

[Geertiebear]

Are demo games supposed to show what the engine is capable of (as in, its best features), or is it supposed to show just the basic things of the engine? Guess what I'm asking is how complex these games will, and are supposed to be. Because if they are more complex it might be a good idea to split them up in their own projects..

[hhirsch]

@Geertiebear I have some other issues for demo games that could be their own project for example #68. The ones I am referring to here should showcase engine features. I am changing the title of this issue accordingly. The main point is that if a radix user wants to build an FPS there is already a demo that shows how the basic mechanic of looking, walking and shooting is implemented. We probably should start with an FPS demo and we will probably add new demos as we develop new features. So #67 is probably a sub-task to this.