Support for 2D monitor layouts

[dfoxfranke]

The syntax of services.xserver.xrandrHeads should be extended to allow lists of lists, to support multihead setups that are arranged in two dimensions. For example, a setting of xrandrHeads = [["DP-0" "DP-2"] ["DP-4" "DP-6"]]; should lead to DP-2 being positioned to the right of DP-0, DP-4 being positioned below DP-0, and DP-6 being positioned to the right of DP-4 and below DP-2.

[stale]

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