Core: Modernize

modernize-use-auto
modernize-use-nullptr
modernize-use-override
modernize-use-using
modernize-use-default-member-init (partially)

Change-Id: Idf10d7ffb1d98a04edc09a25c35e4e9e3abe87b5
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/coreplugin/icore.cpp

@@ -337,8 +337,8 @@ ICore::ICore(MainWindow *mainwindow)
 
 ICore::~ICore()
 {
-    m_instance = 0;
-    m_mainwindow = 0;
+    m_instance = nullptr;
+    m_mainwindow = nullptr;
 }
 
 void ICore::showNewItemDialog(const QString &title,