gui: single instance mode.

3rd/SingleApplication-3.5.2/Windows.md

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+# Windows Specifics
+
+## Setting the foreground window
+
+In the `instanceStarted()` example in the `README` we demonstrated how an
+application can bring it's primary instance window whenever a second copy
+of the application is started.
+
+On Windows the ability to bring the application windows to the foreground is
+restricted, see [AllowSetForegroundWindow()](https://msdn.microsoft.com/en-us/library/windows/desktop/ms632668.aspx) for more details.
+
+The background process (the primary instance) can bring its windows to the
+foreground if it is allowed by the current foreground process (the secondary
+instance). To bypass this `SingleApplication` must be initialized with the
+`allowSecondary` parameter set to `true` .
+
+If the widget is minimized to Windows task bar, `QWidget::raise()` or
+`QWidget::show()` can not bring it to the front, you have to use Windows API
+`ShowWindow()` .
+
+You can find [demo code](examples/windows_raise_widget/main.cpp) in the examples directory.