QMainWindow::showMinimized() doesn't work on Ubuntu 18.04(GNOME Shell)

Showing a GUI application with `showMinmized()` doesn't work on Ubuntu 18.04 and the window is displayed on the screen as if I started it with `showNormal()`.
Note that it works fine on Ubuntu 16.04.

I found that if I execute `mainWindow->showNormal(); mainWindow->sesWindowState(Qt::WindowMinimized);` the window is minimized.

Steps to reproduce this issue:

• open the "Main Window" example
• change `mainWin.show();` in `main.cpp` with `mainWin.showMinimized()`
• recompile and run
  -> the window is visible at start