save list of recently closed playlists for easy undo

I was just thinking (after accidently clicking the 'x' on a playlist tab) that it would be nice to be able to undo this type of mistake (like you can with web browsers.)

The easiest way I can think to implement it w/out changing much code is to have a 'Recently Closed Tabs' folder under the 'Playlists' tab, and when a tab is closed, it gets saved automatically to this folder. This folder can be cleared on shutdown or after N days or whatever, but it could be useful.