Core: Give a hint why saving of settings are requested

Not all requests are the same, handling code might want to act
differently on different request reasons.

Main driver here is the handling of the debugger/analyzer main window
state savings which depends on actual visibility of certain windows.

Change-Id: I87b2a9149e3d09d27bc14b44aace9f2e0686db04
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/coreplugin/mainwindow.cpp

@@ -322,7 +322,7 @@ void MainWindow::closeEvent(QCloseEvent *event)
         return;
     }
 
-    ICore::saveSettings();
+    ICore::saveSettings(ICore::MainWindowClosing);
 
     // Save opened files
     if (!DocumentManager::saveAllModifiedDocuments()) {