Fix -Wimplicit-fallthrough warnings from GCC 8

Adds `FALLTHROUGH` macro to specify when a fallthrough is intentional.
Can be replaced by `[[fallthrough]]` if/when we switch to C++17.

The warning is now enabled by default for GCC on `extra` warnings level
(part of GCC's `-Wextra`). It's not enabled in Clang's `-Wextra` yet,
but we could enable it manually once we switch to C++11. There's no
equivalent feature in MSVC for now.

Fixes #26135.

scene/3d/collision_object.cpp

@@ -52,7 +52,7 @@ void CollisionObject::_notification(int p_what) {
 
 			_update_pickable();
 			//get space
-		};
+		} break;
 
 		case NOTIFICATION_TRANSFORM_CHANGED: {