Defer XAM UI functions & other improvements.

- [Kernel] Fix global locking in kernel dispatch thread.
- [Kernel] Improve CompleteOverlappedDeferred/CompleteOverlappedDeferredEx.
- [XAM] Identify unknowns in XamNotifyCreateListener/XNotifyListener.
- [XAM] Defer XamShowMessageBoxUI.
- [XAM] Defer XamShowKeyboardUI.
- [XAM] Fix cancel handling in XamShowKeyboardUI.
- [XAM] Defer XamShowDeviceSelectorUI.
- [XAM] Defer XamShowDirtyDiscErrorUI.

src/xenia/base/mutex.h

@@ -69,6 +69,11 @@ class global_critical_region {
     return std::unique_lock<std::recursive_mutex>(mutex());
   }
 
+  // Acquires a deferred lock on the global critical section.
+  inline std::unique_lock<std::recursive_mutex> AcquireDeferred() {
+    return std::unique_lock<std::recursive_mutex>(mutex(), std::defer_lock);
+  }
+
   // Tries to acquire a lock on the glboal critical section.
   // Check owns_lock() to see if the lock was successfully acquired.
   inline std::unique_lock<std::recursive_mutex> TryAcquire() {