We used to support calling QEventLoop::exec() from within a processEvent
recursion and still jump down to the root native runloop, but this does
not work as intended due to how QEventDispatcherCoreFoundation uses
flags in its ProcessEventsState for e.g. deferred wake up or timer
updates. The logic in QEventDispatcherCoreFoundation assumes that
the next recursion to processEvents will be handled by itself, so
that it can interpret the flags in ProcessEventsState. The iOS
event dispatcher subclass, QIOSEventDispatcher, does neither of
these things, and should only be used from a 'clean' state.

Change-Id: I44fa156feecc45772806002465c35bef0797ead2
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@theqtcompany.com>