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.

WindowServer/WSWindow.cpp

@@ -118,6 +118,9 @@ void WSWindow::on_message(WSMessage& message)
     case WSMessage::WindowDeactivated:
         gui_event.type = GUI_Event::Type::WindowDeactivated;
         break;
+    case WSMessage::WindowCloseRequest:
+        gui_event.type = GUI_Event::Type::WindowCloseRequest;
+        break;
     default:
         break;
     }