mainwindow: Close window instead of quitting on Ctrl-W etc.

The end result is the same, so this isn't strictly needed - originally
this patch fixed a segfault, but that segfault got fixed in a different
way before merging this patch.

src/mainwindow.cc

@@ -222,7 +222,7 @@ bool MainWindow::on_key_press_event(GdkEventKey* event) {
             case GDK_KEY_Q:
             case GDK_KEY_w:
             case GDK_KEY_q:
-                this->get_application()->quit();
+                close();
                 return true;
         }
     }