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/windows/os_windows.cpp

@@ -39,7 +39,6 @@
 #include "core/version_generated.gen.h"
 #include "drivers/windows/dir_access_windows.h"
 #include "drivers/windows/file_access_windows.h"
-#include "drivers/windows/thread_windows.h"
 #include "joypad_windows.h"
 #include "lang_table.h"
 #include "main/main.h"
@@ -176,8 +175,6 @@ void OS_Windows::initialize() {
 
 	//RedirectIOToConsole();
 
-	ThreadWindows::make_default();
-
 	FileAccess::make_default<FileAccessWindows>(FileAccess::ACCESS_RESOURCES);
 	FileAccess::make_default<FileAccessWindows>(FileAccess::ACCESS_USERDATA);
 	FileAccess::make_default<FileAccessWindows>(FileAccess::ACCESS_FILESYSTEM);