ssd: unify struct ssd_part with struct node_descriptor

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.