Close the current window instead of the main window from the menu bar

Systems with native menu bars show the same menu bar for all child windows. As a result, you cannot assume that the menu bar's parent (the `MainWindow`) will be the window in focus.

launcher/Application.cpp

@@ -80,6 +80,7 @@
 #include <QStringList>
 #include <QDebug>
 #include <QStyleFactory>
+#include <QWindow>
 
 #include "InstanceList.h"
 
@@ -1267,6 +1268,12 @@ bool Application::kill(InstancePtr instance)
     return true;
 }
 
+void Application::closeCurrentWindow()
+{
+    if (focusWindow())
+        focusWindow()->close();
+}
+
 void Application::addRunningInstance()
 {
     m_runningInstances ++;