Editor code: tidy up BOOLs,NULLs and overrides pt4.

A few 'typedefs' replaced by 'using's
This shouldn't have any functional changes at all, just c++17 modernization
It's a part 4 of a split #2847

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

Code/Editor/Util/ImageGif.cpp

@@ -223,7 +223,7 @@ bool CImageGif::Load(const QString& fileName, CImageEx& outImage)
     Pass = 0;
     OutCount = 0;
 
-    Palette = NULL;
+    Palette = nullptr;
     CHK (Raster = new uint8 [filesize]);
 
     if (strncmp((char*) ptr, id87, 6))