Add a configuration option to prompt confirmation when closing tab

[yaslama]

fix #2350

Replace #2380

[wez]

I pushed f2f2b8389 which is a subset of this; just the bits that make middle clicking trigger the prompt according to the established rules for closing tabs with confirmation.

I don't want to add another confirmation option for tabs that bypasses that logic because it makes things even more complex than they already are, diverges the behavior around window and tab closing, and the implementation in this PR has that at the wrong layer: it forces the use of a global configuration rather than the config scoped to the window which makes things a bit more difficult to manage.

What I'd recommend for you is that you set skip_close_confirmation_for_processes_named = {} so that you always get a prompt when you close a tab using the mouse.

[yaslama]

Thanks a lot!! This fixes the issue for me.