gtk: handle all close-requests

When compiling in any Release mode, the default gtk close-request
handler does not properly fire. We only rely on the default handler if
we aren't confirming the close. Replace the default handler with our own
in all cases (and just destroy the window if we don't need
confirmation).

Fixes #458

src/apprt/gtk.zig

@@ -596,7 +596,10 @@ const Window = struct {
             if (surface.window == self) {
                 if (surface.core_surface.needsConfirmQuit()) break;
             }
-        } else return false;
+        } else {
+            c.gtk_window_destroy(self.window);
+            return true;
+        }
 
         // Setup our basic message
         const alert = c.gtk_message_dialog_new(