Shutdown segfault due to SignalManager::clear()

SignalManager::clear() deletes all of the global receiver callbacks without first disconnecting the receiver from the signal it's connected to. This can segfault when the callback is holding a last reference to an QObject is released, causing the QObject to be deleted, and a destroyed signal calls down a now invalid callback. One way to fix it might be just not to make callbacks if the program is shutting down, but a call to SignalManager::clear() before shutdown would still be unsafe.