Detect and abort if an object is deleted during signal handling

Detect and abort if an object is deleted during signal handling

If an object gets deleted while one of its own signal handler
expressions is being evaluated, a subsequent crash is inevitable.

While we could introduce guards/checks in the signal handler kernel
(QMetaObject::activate(), QQmlNotifier::emitNotify() and friends) to
detect and mask the sender deletion, this arguably isn't helpful; the
code that emitted the signal is likely to access member variables
directly after emitting the signal, causing semi-random crashes.

This situation is a symptom of misbehaving application code. Catch it
early rather than later, and issue a qFatal() with a helpful message.
Coupled with a backtrace, this should make it easier to track down
the flawed C++ application logic.

Change-Id: I8c77800e49c475def613224f208181c2d0af60e6
Reviewed-by: Lars Knoll <lars.knoll@nokia.com>
Reviewed-by: Aaron Kennedy <aaron.kennedy@nokia.com>