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.

(cherry picked from commit fc370b3feb)

scene/gui/button.cpp

@@ -29,6 +29,7 @@
 /*************************************************************************/
 
 #include "button.h"
+
 #include "core/translation.h"
 #include "servers/visual_server.h"
 
@@ -102,6 +103,7 @@ void Button::_notification(int p_what) {
 
 					break;
 				}
+				FALLTHROUGH;
 			}
 			case DRAW_PRESSED: {