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/android/api/jni_singleton.h

@@ -83,7 +83,7 @@ public:
 			v = (jvalue *)alloca(sizeof(jvalue) * p_argcount);
 		}
 
-		JNIEnv *env = ThreadAndroid::get_env();
+		JNIEnv *env = get_jni_env();
 
 		int res = env->PushLocalFrame(16);