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/Viewport.cpp

@@ -192,7 +192,7 @@ QtViewport::QtViewport(QWidget* parent)
     m_viewTM.SetIdentity();
     m_screenTM.SetIdentity();
 
-    m_pMouseOverObject = 0;
+    m_pMouseOverObject = nullptr;
 
     m_bAdvancedSelectMode = false;
 
@@ -398,7 +398,7 @@ void QtViewport::OnDeactivate()
 //////////////////////////////////////////////////////////////////////////
 void QtViewport::ResetContent()
 {
-    m_pMouseOverObject = 0;
+    m_pMouseOverObject = nullptr;
 
     // Need to clear visual object cache.
     // Right after loading new level, some code(e.g. OnMouseMove) access invalid