Add an assertion to deleteLater() before it modifies QEvent::d

Add an assertion to deleteLater() before it modifies QEvent::d

The deferred deletion functionality stores the event loop level
nesting count in the QEvent d pointer. In Qt 4, this d pointer was
not usable because we forgot to add a proper copy constructor and
assignment operator to it, so the deleteLater() process stored the
count here safely.

Since Qt 5 now has non-implicit copy methods, the d pointer could be
used in the future. If QEvent uses it, this assertion will
trigger. Note that it doesn't apply to classes derived from QEvent,
though.

Change-Id: I8600c8e9379921e32aca166bc0a6c0b4c4ed799f
Reviewed-by: Bradley T. Hughes <bradley.hughes@nokia.com>