Made all header files to have LABWC_ prefix in include guard identifers.
Converted from __LABWC_ in 35 include/ files.
Converted from __LAB_ in 5 include/ files.
Added LABWC prefix to 3 include/ files.
Added include guards to 3 include/ files.
The double underscores were removed since according to C standard
those "are always reserved for any use".
...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
