The destroyed signal can be received by a deleted receiver

When a widget connects itself to the destroyed signal of a child widget using a functor, the slot code is called when the widget itself is destroyed and after its own destructor has been called. This does not happen when connecting using using the SIGNAL/SLOT macros.

Sample project is included.

To reproduce:

1. Load and run the sample project in QtCreator. Note that the qFatal() call in CustomTopWidget::wasDestroyed will be reached.
2. Change constexpr bool SHOW_BUG = true; to false and rerun. This time the qFatal will not be reached.