ssd: unify struct ssd_part with struct node_descriptor

struct ssd_part and struct node_descriptor seem to have essentially the
same purpose: tag a wlr_scene_node with some extra data indicating what
we're using it for.

Also, as with enum ssd_part_type (now lab_node_type), ssd_part is used
for several types of nodes that are not part of SSD.

So instead of the current chaining (node_descriptor -> ssd_part), let's
flatten/unify the two structs.

In detail:

- First, merge node_descriptor_type into lab_node_type.
- Add a separate view pointer in node_descriptor, since in the case of
  SSD buttons we need separate view and button data pointers.
- Rename ssd_part_button to simply ssd_button. It no longer contains
  an ssd_part as base.
- Add node_try_ssd_button_from_node() which replaces
  node_ssd_part_from_node() + button_try_from_ssd_part().
- Factor out ssd_button_free() to be called in node descriptor destroy.
- Finally, get_cursor_context() needs a little reorganization to handle
  the unified structs.

Overall, this simplifies the code a bit, and in my opinion makes it
easier to understand. No functional change intended.

src/input/cursor.c

@@ -609,7 +609,7 @@ cursor_process_motion(struct server *server, uint32_t time, double *sx, double *
 	struct cursor_context ctx = get_cursor_context(server);
 	struct seat *seat = &server->seat;
 
-	if (ctx.type == LAB_NODE_MENU) {
+	if (ctx.type == LAB_NODE_MENUITEM) {
 		menu_process_cursor_motion(ctx.node);
 		cursor_set(&server->seat, LAB_CURSOR_DEFAULT);
 		return false;
@@ -1173,7 +1173,7 @@ cursor_process_button_release(struct seat *seat, uint32_t button,
 	if (server->input_mode == LAB_INPUT_STATE_MENU) {
 		/* TODO: take into account overflow of time_msec */
 		if (time_msec - press_msec > rc.menu_ignore_button_release_period) {
-			if (ctx.type == LAB_NODE_MENU) {
+			if (ctx.type == LAB_NODE_MENUITEM) {
 				menu_call_selected_actions(server);
 			} else {
 				menu_close_root(server);