Application looks frozen when working with big graphs

[GoogleCodeExporter]

What steps will reproduce the problem?
1. Open application
2. Load a big graph
3.

What is the expected output? What do you see instead?
Application should look like before loading the graph.
Instead the application window turns grey, it gives the appearance that the 
application is frozen but it is not. If the user waits long enough and the 
computation ends the application window looks like it is supposed to look.

Original issue reported on code.google.com by bombax on 1 Mar 2012 at 2:54

[federicobond]

Should probably use the classes from the QtConcurrent namespace to offload work to a background thread and keep the UI responsive. Careful with threading problems when communicating back the results.