Complement function is broken for larger graphs

[xiaoxiae]

The complement function poorly handles larger graphs, making the nodes fly in all directions, causing arithmetic errors.

Steps to reproduce

1. create a large cycle
2. press the Complement button

Possible solutions

The fix should pretty much just involve trying out various functions that repulse/attract the nodes, since the current ones don't seem to handle long distances too well (see __init__.Canvas.repulsion and __init__.Canvas.attraction attributes).