wayland: handle outputs disappearing

That is, deal with monitors being unplugged.

At least on Sway 1.4, surfaces are not unmapped before the output is
removed. Thus, in addition to free:ing the monitor resources, we also
need to update all terminals that are mapped on this output - remove
the output from their "mapped on" list.

wayland.h

@@ -20,6 +20,7 @@ struct monitor {
     struct wl_output *output;
     struct zxdg_output_v1 *xdg;
     char *name;
+    uint32_t wl_name;
 
     int x;
     int y;