Add each view's shell to JSON description

This is a criteria you can use to select windows since commit
484cc189e9 ("Add shell criteria token"), but there's no way to query
it for an existing window. This exposes its value in the output of
`swaymsg -t get_tree`.

sway/ipc-json.c

@@ -493,6 +493,8 @@ static void ipc_json_describe_view(struct sway_container *c, json_object *object
 
 	json_object_object_add(object, "max_render_time", json_object_new_int(c->view->max_render_time));
 
+	json_object_object_add(object, "shell", json_object_new_string(view_get_shell(c->view)));
+
 #if HAVE_XWAYLAND
 	if (c->view->type == SWAY_VIEW_XWAYLAND) {
 		json_object_object_add(object, "window",

sway/sway-ipc.7.scd

@@ -376,6 +376,9 @@ node and will have the following properties:
 |- visible
 :  boolean
 :  (Only views) Whether the node is visible
+|- shell
+:  string
+:  (Only views) The shell of the view, such as _xdg\_shell_ or _xwayland_
 |- window
 :  integer
 :  (Only xwayland views) The X11 window ID for the xwayland view
@@ -672,6 +675,7 @@ node and will have the following properties:
 							"pid": 23959,
 							"app_id": null,
 							"visible": true,
+							"shell": "xwayland",
 							"window_properties": {
 								"class": "URxvt",
 								"instance": "urxvt",
@@ -726,6 +730,7 @@ node and will have the following properties:
 							"pid": 25370,
 							"app_id": "termite",
 							"visible": true,
+							"shell": "xdg_shell",
 							"nodes": [
 							]
 						}