Multiple displays support

[durasj]

@captbaritone mentioned this solution in #494:

[...]the window stick to the edge of the first screen, and has to be pushed a bit farther into the next screen before the change actually happens. When it does cross the threshold, it takes the other windows with it? Basically there's some implied friction/resistance at the boundary.

Just an idea. I'd love to hear others if people have them. One question is: what formation should the non-moving windows have when they move to the new screen? I think ideally they would have the same (or similar) positions to those that they had on the old screen. Obviously we would need to make adjustments if the screen size of the new screen is smaller, but perhaps we could keep it proportional.