Auto fit or center when setting model

[tomvdbussche]

When setting the model (rather than updating) I usually fit it to the screen immediately afterwards. I don't want any animations in this case so I disable animation for the fit to screen action. The issue here is that because these are separate actions, the model briefly appears on top of the screen before drawing it in the middle of the screen.

I would be nice to be able to fit or center the diagram before rendering it to the screen.

[spoenemann]

Related: #147