fix: ignore save requests after closing all windows

[Nerixyz]

Fixes #1098. WindowManager::save is called two times when closing all windows:

1. When closing a main window:

   chatterino2/src/widgets/Window.cpp

   Lines 135 to 142 in 1554d7b

   	void Window::closeEvent(QCloseEvent *)
   	{
   	if (this->type_ == WindowType::Main)
   	{
   	auto app = getApp();
   	app->windows->save();
   	app->windows->closeAll();
   	}

2. When saving all singletons:

   chatterino2/src/Application.cpp

   Lines 335 to 341 in 1554d7b

   	void Application::save()
   	{
   	for (auto &singleton : this->singletons_)
   	{
   	singleton->save();
   	}
   	}

The second save doesn't see the closed child windows if all windows were closed with closeAll. So after closeAll is called, the window manager will now ignore all save events.