TCA asks for confirmation if you close it during progress

!444 (merged) closed #18303 (closed) but introduced a situation in which there is bad UX.

Here it is:

• The user is connected to a wifi network that blocks all connections to Tor
• the user selects "easy mode"
• the progress bar appears and lets the user waiting
• the user closes the window because they are bored
• after a while, the user notices they can't browse the web
• they open TCA again. Now, TCA can't know how much time was spent trying to configure the network. So the best TCA can do is showing the progress bar from the start and letting the user wait other 30 seconds.

To mitigate this usability bug, this MR introduces a confirmation dialog on close. It only appears when there is a visible progressbar.

This MR depends on !445 (merged)