Default project when cancelling project loading (#3941)

Default project when cancelling project loading

If a user isn't aware that the loading of a project has been
cancelled, it may be in an incomplete state. Saving such a project
will overwrite the original file and result in data loss. This is
solved by loading the default project on cancelling project loading.

Add Mixer::clearNewPlayHandles() to prevent crash when cancelling
loading of a single streamed instrument.

include/Mixer.h

@@ -155,6 +155,7 @@ public:
 
 	void initDevices();
 	void clear();
+	void clearNewPlayHandles();
 
 
 	// audio-device-stuff

include/Song.h

@@ -33,6 +33,7 @@
 #include "TrackContainer.h"
 #include "Controller.h"
 #include "MeterModel.h"
+#include "Mixer.h"
 #include "VstSyncController.h"
 
 
@@ -229,6 +230,17 @@ public:
 		return m_loadingProject;
 	}
 
+	void loadingCancelled()
+	{
+		m_isCancelled = true;
+		Engine::mixer()->clearNewPlayHandles();
+	}
+
+	bool isCancelled()
+	{
+		return m_isCancelled;
+	}
+
 	bool isModified() const
 	{
 		return m_modified;
@@ -358,6 +370,7 @@ private:
 	volatile bool m_paused;
 
 	bool m_loadingProject;
+	bool m_isCancelled;
 
 	QStringList m_errors;