ssd: clean up scene management

Our codebase for ssd scenes has grown with a lot of technical debts:
- We needed to call `ssd_get_part()` everywhere to get the scene node of a
  ssd part. We then needed to cast it to `wlr_scene_rect` and
  `wlr_scene_buffer`. This bloated our codebase and even blocked
  duplicated button types in `<titlebar><layout>`.
- `ssd_get_part_type()` was a dirty hack. It compared parent, grandparent
  and grandgrandparent of a node with each subtree in the ssd to get the
  part type of the node.

To resolve this issues, this commit changes how ssd scenes are managed:
- Access scene rects and scene buffers just as a member of `struct ssd`.
- `ssd_part` is now a attachment to a scene node that can be accessed via
  node_descriptor->data, with a new node-descriptor type
  `LAB_NODE_DESC_SSD_PART`. `LAB_NODE_DESC_SSD_BUTTON` is unified into it.

Now the scene graph under ssd->tree looks like below. The parentheses
indicate the type of ssd_part attached to the node:

ssd->tree (LAB_SSD_NONE)
+--titlebar (LAB_SSD_PART_TITLEBAR)
|  +--inactive
|  |  +--background bar
|  |  +--left corner
|  |  +--right corner
|  |  +--title (LAB_SSD_PART_TITLE)
|  |  +--iconify button (LAB_SSD_BUTTON_ICONIFY)
|  |  |  +--normal close icon image
|  |  |  +--hovered close icon image
|  |  |  +--...
|  |  +--window icon (LAB_SSD_BUTTON_WINDOW_ICON)
|  |  |  +--window icon image
|  |  +--...
|  +--active
|     +--...
+--border
|  +--inactive
|  |  +--top
|  |  +--...
|  +--active
|     +--top
|     +--...
+--shadow
|  +--inactive
|  |  +--top
|  |  +--...
|  +--active
|     +--top
|     +--...
+--extents
   +--top
   +--...

When hovering on SSD, `get_cursor_context()` traverses this scene node
from the leaf. If it finds a `ssd_part` attached to the node, it returns
`ssd_part_type` that represents the resizing direction, button types or
`Title`/`Titlebar`.

src/config/rcxml.c

@@ -155,6 +155,7 @@ fill_section(const char *content, struct wl_list *list, uint32_t *found_buttons)
 
 		assert(type != LAB_SSD_NONE);
 
+		/* We no longer need this check, but let's keep it just in case */
 		if (*found_buttons & (1 << type)) {
 			wlr_log(WLR_ERROR, "ignoring duplicated button type '%s'",
 				identifier);