Hidden/closed dialogs receive input events

Closed dialogs still accept queued input events which can cause accepted() and finished() signals to fire multiple times. For example when there is some time consuming processing blocking the main thread, then user tries to accept a dialog multiple times.

This can be worked around relatively easily by always disabling/disconnecting, but that requires user to always do that basically for every dialog. Therefore, it might be better to drop input events for hidden dialogs/widgets.