DSPDevice*Engine: Add signals to indicate when commands have been processed.
DSPDeviceSourceEngine: Fix small memory leak.
DSPEngine::removeDeviceEngineAt: Remove wait to avoid blocking thread. Return QThread to get finished signal.
DSPEngine::addDevice*Engine: Don't call deleteLater for device*Engine, as these objects are deleted manually in MainWindow, which will crash if deleteLater called first.
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