Implement thread_ctrl::is_main()

Utilities/Thread.h

@@ -342,6 +342,9 @@ public:
 	// Get thread ID (works for all threads)
 	static u64 get_tid();
 
+	// Check whether current thread is main thread (usually Qt GUI)
+	static bool is_main();
+
 private:
 	// Miscellaneous
 	static const u64 process_affinity_mask;