Core: Fix that core settings like shortcuts were only saved at shutdown

Save them also whenever a settings save is requested, which e.g. also
happens when closing the options dialog.

Change-Id: Ic20f1007381a4d0a8157f17fd24c0d8ea24907ee
Task-number: QTCREATORBUG-16576
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/coreplugin/mainwindow.h

@@ -111,6 +111,8 @@ public:
 
     void addPreCloseListener(const std::function<bool()> &listener);
 
+    void saveSettings();
+
 signals:
     void newItemDialogRunningChanged();
 
@@ -149,7 +151,7 @@ private:
     void registerDefaultActions();
 
     void readSettings();
-    void writeSettings();
+    void saveWindowSettings();
 
     ICore *m_coreImpl;
     QStringList m_aboutInformation;