completion: reduce flapping of completion visibility
completion: reduce flapping of completion visibility

This tries to avoid a flapping situation with completion lists where we
might get an initial notify::empty and then very shortly thereafter get
a notify::empty again with a different value.

When this would happen, we'd hide/show the completion assistant within the
same frame cycle, yet resources would have already been discarded from the
previous call to gtk_widget_hide().

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