Web eID dialog disposing

TODO: Add a diagram that depicts the call and event chains that cause the Web eID dialog to be disposed and deleted.

Dialog disposal is quite complex as there are a number of different scenarios that cause the disposal:

1. retriable error occurs, user clicks Retry,
2. fatal error occurs,
3. user clicks Cancel on the dialog (window destroyed → controller exit),
4. user clicks OK on the dialog (window destroyed → controller exit),
5. user clicks Cancel on the PIN pad (window destroyed → controller exit),
6. user clicks OK on the PIN pad (window destroyed → controller exit).

Connecting the window destroyed signal to the controller exit slot assures that the window has been properly disposed when the controller exits.