Set a fixed bond length to reduce jumping when adding atoms

[jacktday]

This reduces jumping when adding atoms by reducing the need to resize the screen. This could be further improved by preventing the renderer from centering the molecule. Centering should already be turned off, there is likely some other setting that needs to be changed.

[jacktday]

This now prevents any jumping, but disables auto scaling. I could manually add auto scaling or a way of panning and zooming could be added to the UI. I think the later is a better solution

[EBjerrum]

Thanks a lot for all you pull request! Unfortunately I'm super busy these final days before Christmas, but hopefully I will find the time to look at them soon