shared_object: Use deleteLater in unref for thread safety

This makes unref safe when it's not called from within the object's thread.

Fixes #4009

Author: lukas-w

include/shared_object.h

@@ -61,7 +61,7 @@ class sharedObject
 
 		if ( deleteObject )
 		{
-			delete object;
+			object->deleteLater();
 		}
 	}