The outputOnly property allows to specify that the dialog should not
accept any input. Thus it's an output only window which supports click
through. This is obviously platform specific and so far it is only
implemented for the X11 platform using the shape extension.
The input shape needs to be set once the window is visible and thus
the functionality is bound to the visible changed signal. The code
ensures that the required shape extension version is present and only
fetches it once.
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