Fix startup tips GUI to single height and width

[Jelmergu]

After the update to 0.16 the startup tips window appeared again after loading a game(not much of a problem) and I noticed that the next button position appears to be moving up and down depending on the tip that is shown.

Personally I don't like to be moving my mouse when I have a window with a next button. I think it is better to have the ability to have the mouse hover over the next button and after reading have the ability to just click.

It is only a minor issue in my opinion, as most players will probably only see the GUI once