implement display_sort_order config option

[jneen]

Alternate proposal to #551 .

Usage: yabai -m config display_sort_order [none|horizontal|vertical] (default: none)

If set to something other than none, display arrangement indexes will be considered relative to their position, either sorted horizontally or vertically. This also affects the behaviour of the positional selectors next, prev, first, and last, making it ideal for #225 .

[jneen]

Rebased. It appears some similar functionality has been added in the meanwhile, but I still think it's useful to be able to indicate display numbers relative to their position.

[jneen]

Rebased and squashed. Additionally, if horizontal or vertical are used, equally positioned monitors will attempt to fall back to the other axis. That way, situations like:

[  A  ]
[  B  ] [ C ]

will correctly sort the monitors as [A, B, C].

[jneen]

I've gone ahead and rebased this again.

[jneen]

rebased.

[koekeishiya]

Implemented on master e02addf.

display_arrangement_order [default|vertical|horizontal]

Specify how displays are ordered (determined by center point).
default: Native macOS ordering.
vertical: Order by y-coordinate (followed by x-coordinate when equal).
horizontal: Order by x-coordinate (followed by y-coordinate when equal).