Add convenient C++ enums for thread priorities

```c++
enum class ThreadPriority : KPRIORITY {
Low = LOW_PRIORITY,
LowRealtime = LOW_REALTIME_PRIORITY,
High = HIGH_PRIORITY,
Maximum = MAXIMUM_PRIORITY
};

enum class PriorityBoost: KPRIORITY {
NoBoost = IO_NO_INCREMENT,
OnEvent = EVENT_INCREMENT,
OnSemaphore = SEMAPHORE_INCREMENT,
Video = IO_VIDEO_INCREMENT,
Parallel = IO_PARALLEL_INCREMENT,
Serial = IO_SERIAL_INCREMENT,
CdRom = IO_CD_ROM_INCREMENT,
Disk = IO_DISK_INCREMENT,
MailSlot = IO_MAILSLOT_INCREMENT,
NamedPipe = IO_NAMED_PIPE_INCREMENT,
Network = IO_NETWORK_INCREMENT,
Keyboard = IO_KEYBOARD_INCREMENT,
Mouse = IO_MOUSE_INCREMENT,
Sound = IO_SOUND_INCREMENT
};
```