Added a command for automatically reloading screens when their ui file changed.

The command is called enableAutoScreenReloading.

Note that in the current intelij project setup lets terasology load screens from engine/build/classes/assets/ui/.

To perform a live editing of the container screen you need to edit engine/build/classes/assets/ui/ingame/containerScreen.ui

@immortius  is it purpose that the files get copied to the target directory?

It would be cool if terasology could use the source files directly instead of copying them to a classes directory first.