Utils: Modernize further

Many issues, mostly in headers, were not addressed in
e38410b76c

modernize-use-auto
modernize-use-nullptr
modernize-use-override
modernize-use-using
modernize-use-default-member-init
modernize-use-equals-default

Change-Id: I320a51726db881e582b898948d53735ebb06887a
Reviewed-by: hjk <hjk@qt.io>

src/libs/utils/treeviewcombobox.cpp

@@ -44,7 +44,7 @@ void TreeViewComboBoxView::adjustWidth(int width)
 
 
 TreeViewComboBox::TreeViewComboBox(QWidget *parent)
-    : QComboBox(parent), m_skipNextHide(false)
+    : QComboBox(parent)
 {
     m_view = new TreeViewComboBoxView;
     m_view->setHeaderHidden(true);