Editor code: tidy up BOOLs,NULLs and overrides pt3.

A few 'typedefs' replaced by 'using's
This shouldn't have any functional changes at all, just c++17 modernization
It's a part 3 of a split #2847

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

Code/Editor/QtUI/QCollapsibleGroupBox.cpp

@@ -13,7 +13,7 @@
 QCollapsibleGroupBox::QCollapsibleGroupBox(QWidget* parent)
     : QGroupBox(parent)
     , m_collapsed(false)
-    , m_toggleButton(0)
+    , m_toggleButton(nullptr)
 {
     m_toggleButton = new QToolButton(this);
     m_toggleButton->setFixedSize(16, 16);