sway: Add non-desktop-output type

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`

include/sway/tree/root.h

@@ -28,6 +28,7 @@ struct sway_root {
 	double width, height;
 
 	list_t *outputs; // struct sway_output
+	list_t *non_desktop_outputs; // struct sway_output_non_desktop
 	list_t *scratchpad; // struct sway_container
 
 	// For when there's no connected outputs