Editor code: tidy up BOOLs,NULLs and overrides pt6.

A few 'typedefs' replaced by 'using's
This shouldn't have any functional changes at all, just c++17 modernization
It's a part 6 of a split #2847

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

Code/Editor/Plugin.cpp

@@ -133,7 +133,7 @@ IClassDesc* CClassFactory::FindClass(const char* pClassName) const
 
     if (!pSubClassName)
     {
-        return NULL;
+        return nullptr;
     }
 
     QString name = QString(pClassName).left(pSubClassName - pClassName);
@@ -169,7 +169,7 @@ void CClassFactory::UnregisterClass(const char* pClassName)
 {
     IClassDesc* pClassDesc = FindClass(pClassName);
 
-    if (pClassDesc == NULL)
+    if (pClassDesc == nullptr)
     {
         return;
     }