Currently, when encountering a non-desktop display, sway offers the
output for leasing and returns without storing it in a sway specific
output type like `struct sway_output`. Additionally, running
`swaymsg -t get_outputs` doesn't show non-desktop outputs.
This commit stores the non-desktop outputs into a struct called
`sway_output_non_desktop`, and adds them to a list on `sway_root`