Revisit the window size/position API

[jneem]

We currently have a pretty low-level API for positioning windows. It uses coordinates of a virtual screen that contains all the monitors, and (as of [#1713]) positions windows using display points on that virtual screen.

In that discussion, @cmyr suggested that it might be nicer to have coordinates that specify monitor + offset. (For example, because macOS might not allow windows to span multiple monitors, and also because different monitors might want to have different scale factors.)

Also, note that wayland doesn't allow absolute positioning at all, and so we might want an API for positioning subwindows relative to other windows.

Finally, when the default positioning of (e.g.) dropdowns would have them go outside the screen, we might want a better API for specifying how they should be positioned.