Modernize Thread

- Based on C++11's `thread` and `thread_local`
- No more need to allocate-deallocate or check for null
- No pointer anymore, just a member variable
- Platform-specific implementations no longer needed (except for the few cases of non-portable functions)
- Simpler for `NO_THREADS`
- Thread ids are now the same across platforms (main is 1; others follow)

platform/linuxbsd/joypad_linux.h

@@ -72,7 +72,7 @@ private:
 
 	bool exit_udev;
 	Mutex joy_mutex;
-	Thread *joy_thread;
+	Thread joy_thread;
 	Input *input;
 	Joypad joypads[JOYPADS_MAX];
 	Vector<String> attached_devices;