layer: use scene-graph API more fully

layer: use scene-graph API more fully

...and thus simplify the usage of wlr_scene_node_at(). Specifically:

- desktop.c: in get_cursor_context() use node-description for
  layer-surfaces and layer-popups. This lays the foundations for a
  pointer-enter-event being sent when a new layer-surfaces appears under
  the pointer (even if the pointer doesn not move).

- layers.c:

  * Iterate over `struct wlr_scene_tree *layer_tree[]` rather than
    `struct wl_list layers[]` when arranging layers to avoid surfaces
    being out of sync with nodes

  * Set signal handlers after scene node creation to avoid configure
    race conditions

  * Handle scene-node destroy event rather than event of
    `struct wlr_layer_surface_v1`

  * Arrange layers on map and unmap

  * Handle client request for layer-change

Fixes issue #667