Get broadest Rectangle instead of first one in setPosition

Suppose there are 2 screens:

```haskell
> :m + Graphics.X11.Xlib Graphics.X11.Xinerama
> openDisplay "" >>= getScreenInfo >>= print
[Rectangle {rect_x = 0, rect_y = 0, rect_width = 1024, rect_height = 768},Rectangle {rect_x = 0, rect_y = 0, rect_width = 2560, rect_height = 1440}]
```
`Window.setPosition` will return the first one because of strange `head` usage. In my opinion the screen with maximum width could be more adequate choice.