mainapplication: on tray icon click close window if open and focused

Only do so on non-Windows platforms, though; apparently this is not a
common pattern for desktop applications on Windows.

Change-Id: Ie1f8f4dd087d4e2798be4d102eb1d421fa9490f7

src/mainapplication.cpp

@@ -492,8 +492,17 @@ MainApplication::initSystray()
     connect(systemTray_.get(),
             &QSystemTrayIcon::activated,
             [this](QSystemTrayIcon::ActivationReason reason) {
-                if (reason != QSystemTrayIcon::ActivationReason::Context)
+                if (reason != QSystemTrayIcon::ActivationReason::Context) {
+#ifdef Q_OS_WINDOWS
                     restoreApp();
+#else
+                    QWindow* window = focusWindow();
+                    if (window)
+                        window->close();
+                    else
+                        restoreApp();
+#endif
+                }
             });
 
     systrayMenu->addAction(quitAction);