[Threading] Add thread priority mapping with timer-driven quantum decay

And default ignore_thread_priorities to false.

Map Xenon's 0-31 priority range across all 5 host priority levels
instead of collapsing 0-17 into kNormal.

Use timer-driven quantum decay (~20ms period) matching Xenon's
60-quantum / 3-per-tick cycle to prevent starvation by gradually lowering
effective priority for non-real-time threads (< 18), piggybacking on the
existing 1ms timestamp timer.

src/xenia/kernel/kernel_state.h

@@ -384,6 +384,7 @@ class KernelState {
   BitMap tls_bitmap_;
   uint32_t ke_timestamp_bundle_ptr_ = 0;
   std::unique_ptr<xe::threading::HighResolutionTimer> timestamp_timer_;
+  uint32_t quantum_timer_counter_ = 0;
   cpu::backend::GuestTrampolineGroup kernel_trampoline_group_;
   // fixed address referenced by dashboards. Data is currently unknown
   uint32_t strange_hardcoded_page_ = 0x8E038634 & (~0xFFFF);