Support embedding Qt Widgets in non-widget window hierarchies

QtWidget has some fundamental issues relating to being embedded in foreign/non-widget UI hierarchies.

We currently assume that a widget is either:

1. Top level
   • Has Qt::Window flag
   • isWindow() returns true
   • Does not have a QWidget::parent()
   • Has windowHandle(), after being shown
2. Child
   • Does not have Qt::Window (only Qt::Widget) flag
   • isWindow() returns false
   • Has QWidget::parent()
   • Possibly has windowHandle(), if winId() has been called

Based on the documentation of isWindow() :

A window is a widget that isn't visually the child of any other widget and that usually has a frame and a window title.

An embedded child widget should fall into the second case, a child widget, ie not return true for isWindow, not have Qt::Window, etc.

But in a lot of code paths we assume that there is then a QWidget::parent(), which there is not for an embedded child widget, causing crashes in various places.

For now, we work around it by letting embedded widgets still have Qt::Window/isWindow(), but this causes other issues, e.g:

• Being listed in QApplication::topLevelWidgets()
  • Which in turn prevents quit if widget is visible
• Widgets using the child window as transient parent for popups and dialogs (since it's "top level")
  • Resulting in warnings from QWindow::setTransientParent and failure to set transient parent