Connect to signals of the shown window

[Gliese852]

In the case of OnboardingWindow and MainWindow, the class itself is not shown as a window, but its window field is shown, so when the window is closed, the application does not terminate - the wrong signals is connected to the handler.

[Gliese852]

I corrected it a little, because it is not always called with all the args. (An error occurred when clicking the "Cancel" button in the wizard.)