Editor code: tidy up BOOLs,NULLs and overrides pt1.

A few 'typedefs' replaced by 'using's
This shouldn't have any functional changes at all, just c++17 modernization
It's a part 1 of a split #2847

Signed-off-by: Nemerle <nemerle5+git@gmail.com>

Code/Editor/Include/IAssetItem.h

@@ -334,12 +334,12 @@ struct IAssetItem
     virtual void OnEndPreview() = 0;
     // Description:
     //      If the asset has a special preview panel with utility controls, to be placed at the top of the Preview window, it can return an child dialog window
-    //      otherwise it can return NULL, if no panel is available
+    //      otherwise it can return nullptr, if no panel is available
     // Arguments:
-    //      pParentWnd - a valid CDialog*, or NULL
+    //      pParentWnd - a valid CDialog*, or nullptr
     // Return Value:
     //      A valid child dialog window handle, if this asset wants to have a custom panel in the top side of the Asset Preview window,
-    //      otherwise it can return NULL, if no panel is available
+    //      otherwise it can return nullptr, if no panel is available
     // See Also:
     //      OnBeginPreview(), OnEndPreview()
     virtual QWidget* GetCustomPreviewPanelHeader(QWidget* pParentWnd) = 0;