customizable port and node default sizes

[i-make-robots]

Is your feature request related to a problem? Please describe.
snap to grid in donatello would satisfy more if the nodes were exactly grid units wide.

Describe the solution you'd like
use a tweakable number instead of the hard-coded default.

Describe alternatives you've considered

• Resize the grid to match the node size. Done.

Additional context

• Would also mean updating DonatelloSettings and DonatelloSettingsPanel and tests.
• It would be nice to remove dimensions from nodegraphcore completely and introduce them in Donatello, at the graphics level. Unfortunately this creates all kinds of code complexity. nodegraphcore splits Port into Input and Output. Donatello would like to create a VisualPort that adds the rectangle back in, but if I extend Port then how do I also extend Input and Output?