Web API: use message passing to start/stop device for all the rest of device plugins

plugins/samplesource/fcdpro/fcdprogui.h

@@ -55,6 +55,7 @@ private:
 	Ui::FCDProGui* ui;
 
 	DeviceUISet* m_deviceUISet;
+	bool m_doApplySettings;
 	bool m_forceSettings;
 	FCDProSettings m_settings;
 	QTimer m_updateTimer;
@@ -66,6 +67,7 @@ private:
 	int m_lastEngineState;
 	MessageQueue m_inputMessageQueue;
 
+	void blockApplySettings(bool block) { m_doApplySettings = !block; }
 	void displaySettings();
 	void sendSettings();
 	void updateSampleRateAndFrequency();