core/log: Rename LOG_* to CORE_* so that they don't get mixed up by Urho3D's LOG_* constants
doc/conventions.txt: find_x: Returns nullptr or equivalent if not found
doc/conventions.txt: Switch get_x behavior to be like check_x; TODO: Implement this behavior
builtin/main_context, builtin/replicate: New multi-scene handling
builtin/voxelworld: Update to new scene handling
builtin/ground_plane_lighting: Update to new scene handling
games/digger: Update to new scene handling