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.

editor/editor_node.cpp

@@ -1900,7 +1900,8 @@ void EditorNode::_menu_option_confirm(int p_option, bool p_confirmed) {
 				break;
 			}
 
-		} // fallthrough
+			FALLTHROUGH;
+		}
 		case SCENE_TAB_CLOSE:
 		case FILE_SAVE_SCENE: {
 
@@ -1919,8 +1920,8 @@ void EditorNode::_menu_option_confirm(int p_option, bool p_confirmed) {
 
 				break;
 			}
-			// fallthrough to save_as
-		};
+			FALLTHROUGH;
+		}
 		case FILE_SAVE_AS_SCENE: {
 			int scene_idx = (p_option == FILE_SAVE_SCENE || p_option == FILE_SAVE_AS_SCENE) ? -1 : tab_closing;