Graphs should be shown in the center of the window

[buddekai]

Right now, when clicking on "show provenance graph" several times, the graph moves to the right and eventually disappears. This should be circumvented by always centering everything (in the best case without ouverlap) in the center of the window.

[jsmith]

I think this is a bug sadly. I remember this happening last summer and I was never able to reproduce the issue. I'll try again sometime soon though!