Added branch level-editor

[xoniuqe]

I added a new branch called "level-editor" in which I want to improve the current ingame editor, which has nearly no functionality.
Things we need:

• better commands / UI: the current editor commands are pretty limited and poorly presented
• entity editor + placement : add a window were we can create new entity bluebrints and add them to the level
• trigger(?) zones: it would be nice to add zones to the map which fire events or trigger a script (no idea how exactly this could be done)
• maybe an trigger editor?
• maybe an additional mission file in which we can script some map independent but level concerning things

We could also discuss a seperation of mission scripting and map building, so a map could be reusable for multiple missions.