Fix handling of multiple bindings to the same signal or event

 * Fix bug where QtSignalForwarder::bind() would replace existing
   bindings for the same receiver.

 * When connecting a signal with bind(), avoid calling
   QMetaObject::connect() if the signal is already connected.

 * When handling a signal in QtSignalForwarder::qt_metacall(),
   iterate over all matching bindings, not just the first.