Add method to get whether current thread is non-main

src/thread/thread_morphos.cpp

@@ -199,5 +199,6 @@ private:
 void SetSelfAsMainThread() { }
 
 bool IsMainThread() { return false; }
+bool IsNonMainThread() { return false; }
 
 int GetThreadName(char *str, const char *last) { return 0; }