Outline improvements

Hello Lorenzo, 

I have improved the outline a little bit:

- The imports are skipped.
- The aux functions are grouped below their description.
- The rules and axioms are grouped below their judgment description.

Even linking from the editor back into the outline works - although that was some work because now the outline tree differs from the containment tree.

I'd appreciate it, if you can apply these changes also upon v1.5.2, so we can use them with Xtext 2.6.x.

Regards,
Andreas H.