Prompt to reencrypt all before closing application

[JeffreyBenjaminBrown]

I love cryptkeeper. Thank you for it!

It is possible for a user -- at least this one -- to (after closing the application) believe the relevant folders are now inaccessible, but in fact they are left open. If a popup window asked them whether to do that when they tried to quit, it would be safer for the kind of user who makes that assumption.