More work on focus, and good docs

Source/Integration/RootCanvas.h

@@ -19,6 +19,7 @@
 
 #include "CoreMinimal.h"
 #include "Common.h"
+#include "UObject/ObjectKey.h"
 #include "Blueprint/UserWidget.h"
 #include "Components/CanvasPanel.h"
 #include "Components/CanvasPanelSlot.h"
@@ -128,6 +129,9 @@ public:
 		meta = (DefaultToSelf = "Widget", ExpandEnumAsExecs = "Result"))
 	static UlxRootCanvasSlot *GetRootCanvasSlot(class UUserWidget *Widget, ElxSuccessOrWrongType &Result);
 
+	// The last widget whose focus request was granted.
+	TObjectKey<UWidget> LastWidgetGrantedFocus;
+
 protected:
 
 	// We inherit most of our code from CanvasPanel.   This causes the