Move audio fade threads to generic _helperThreads of macro

This will allow waiting for and stopping audio fades when calling Stop()
for a particular macro

src/headers/macro-action-audio.hpp

@@ -16,7 +16,6 @@ enum class AudioAction {
 class MacroActionAudio : public MacroAction {
 public:
 	MacroActionAudio(Macro *m) : MacroAction(m) {}
-	virtual ~MacroActionAudio();
 	bool PerformAction();
 	void LogAction();
 	bool Save(obs_data_t *obj);
@@ -40,7 +39,6 @@ private:
 	void StartMasterFade();
 	void FadeSourceVolume();
 	void FadeMasterVolume();
-	std::thread _fadeThread;
 
 	static bool _registered;
 	static const std::string id;

src/headers/macro.hpp

@@ -88,6 +88,7 @@ public:
 	bool MatchOnChange() { return _matchOnChange; }
 	int GetCount() { return _count; };
 	void ResetCount() { _count = 0; };
+	void AddHelperThread(std::thread &&);
 	bool GetStop() { return _stop; }
 	void Stop();
 	std::deque<std::shared_ptr<MacroCondition>> &Conditions()
@@ -136,7 +137,8 @@ private:
 	bool _die = false;
 	bool _stop = false;
 	bool _done = true;
-	std::thread _thread;
+	std::thread _backgroundThread;
+	std::vector<std::thread> _helperThreads;
 };
 
 Macro *GetMacroByName(const char *name);