[Qgis-developer] Diagram improvements - Merged to master

[Matthias Kuhn]

I'm happy to announce, that my first bigger request has been accepted.
It has been merged into master and I've just closed all the tickets
related to it.

As far as I can see, everything from the old "Overlay" implementation
should be available in the new one as well. As soon as a few people give
a positive feedback on we'll be able to remove the old implementation.

If you have any comments, I'll be happy to read them here and/or in a
bugreport.

Short list of important changes
- Printing works
- Diagrams can overlap each other (smallers on top)
- Histogram diagrams
- GUI has been reordered (elements are hidden when not available)
- Circles are scaled by area (option to switch back to diameter)

Cheers,
Matthias