Exit/quit semantics should close /all/ windows, not just the one

We have the close item for this now.

qcachegrind/qcgtoplevel.cpp

@@ -378,7 +378,7 @@ void QCGTopLevel::createActions()
     _exitAction->setMenuRole(QAction::QuitRole);
     _exitAction->setShortcut(tr("Ctrl+Q"));
     _exitAction->setStatusTip(tr("Exit the application"));
-    connect(_exitAction, &QAction::triggered, this, &QWidget::close);
+    connect(_exitAction, &QAction::triggered, this, &QApplication::closeAllWindows);
 
     // view menu actions