Add a simple close button ("X") to windows.

Clicking the button generates a WindowCloseRequest event which the client app
then has to deal with. The default behavior for GWindow is to close() itself.

I also added a flag, GWindow::should_exit_event_loop_on_close() which does
what it sounds like it does.

This patch exposed some bugs in GWindow and GWidget teardown.

Terminal/main.cpp

@@ -149,6 +149,8 @@ int main(int, char**)
                 terminal.set_in_active_window(true);
             } else if (event.type == GUI_Event::Type::WindowDeactivated) {
                 terminal.set_in_active_window(false);
+            } else if (event.type == GUI_Event::Type::WindowCloseRequest) {
+                return 0;
             }
         }
     }